After having received the 2.3.1 notification, I started the upgrade process a few hours ago.

Here's all the log I was able to receive :

>>> Updating repositories metadata... 
Updating pfSense-core repository catalogue...
pfSense-core repository is up-to-date.
Updating pfSense repository catalogue...
pfSense repository is up-to-date.
All repositories are up-to-date.
>>> Upgrading pkg... done.
>>> Updating repositories metadata... 
Updating pfSense-core repository catalogue...
Fetching meta.txz: . done
Fetching packagesite.txz: . done
Processing entries: .. done
pfSense-core repository update completed. 13 packages processed.
Updating pfSense repository catalogue...
Fetching meta.txz: . done
Fetching packagesite.txz: .......... done
Processing entries: .......... done
pfSense repository update completed. 376 packages processed.
>>> Upgrading pfSense-repo... done.
>>> Updating repositories metadata... 
Updating pfSense-core repository catalogue...
Repository pfSense-core has a wrong packagesite, need to re-create database
Fetching meta.txz: . done
Fetching packagesite.txz: . done
Processing entries: .. done
pfSense-core repository update completed. 13 packages processed.
Updating pfSense repository catalogue...
Repository pfSense has a wrong packagesite, need to re-create database
Fetching meta.txz: . done
Fetching packagesite.txz: .......... done
Processing entries: .......... done
pfSense repository update completed. 376 packages processed.
**** WARNING ****
Duplicate slice required!!

Before starting the upgrade process, the currently mounted nanobsd partition
needs to be cloned to the secondary partition, where the update will happen

After installation a reboot will be required to switch partition.

>>> Cleaning secondary partition... done.
>>> Duplicating current slice... done.
>>> Restoring slice label... done.
>>> Testing duplicated partition integrity... done.
>>> Mounting second partition to run upgrade... done.
>>> Unlocking package pfSense-kernel-pfSense_wrap... done.
>>> Downloading upgrade packages... 
Updating pfSense-core repository catalogue...
pfSense-core repository is up-to-date.
Updating pfSense repository catalogue...
pfSense repository is up-to-date.
All repositories are up-to-date.
Checking for upgrades (61 candidates): .......... done
Processing candidates (61 candidates): .......... done
The following 63 package(s) will be affected (of 0 checked):

New packages to be INSTALLED:
	uclcmd: 0.1 [pfSense]
	libucl: 0.8.0 [pfSense]

Installed packages to be UPGRADED:
	unbound: 1.5.5 -> 1.5.8 [pfSense]
	sqlite3: 3.9.2 -> 3.11.1 [pfSense]
	smartmontools: 6.4_1 -> 6.4_2 [pfSense]
	rate: 0.9 -> 0.9_1 [pfSense]
	python27: 2.7.11_1 -> 2.7.11_2 [pfSense]
	php56-zlib: 5.6.20 -> 5.6.21 [pfSense]
	php56-xmlwriter: 5.6.20 -> 5.6.21 [pfSense]
	php56-xmlreader: 5.6.20 -> 5.6.21 [pfSense]
	php56-xml: 5.6.20 -> 5.6.21 [pfSense]
	php56-tokenizer: 5.6.20 -> 5.6.21 [pfSense]
	php56-sysvshm: 5.6.20 -> 5.6.21 [pfSense]
	php56-sysvsem: 5.6.20 -> 5.6.21 [pfSense]
	php56-sysvmsg: 5.6.20 -> 5.6.21 [pfSense]
	php56-sqlite3: 5.6.20 -> 5.6.21 [pfSense]
	php56-sockets: 5.6.20 -> 5.6.21 [pfSense]
	php56-simplexml: 5.6.20 -> 5.6.21 [pfSense]
	php56-shmop: 5.6.20 -> 5.6.21 [pfSense]
	php56-session: 5.6.20 -> 5.6.21 [pfSense]
	php56-readline: 5.6.20 -> 5.6.21 [pfSense]
	php56-posix: 5.6.20 -> 5.6.21 [pfSense]
	php56-pdo_sqlite: 5.6.20 -> 5.6.21 [pfSense]
	php56-pdo: 5.6.20 -> 5.6.21 [pfSense]
	php56-pcntl: 5.6.20 -> 5.6.21 [pfSense]
	php56-openssl: 5.6.20 -> 5.6.21 [pfSense]
	php56-opcache: 5.6.20 -> 5.6.21 [pfSense]
	php56-mcrypt: 5.6.20 -> 5.6.21 [pfSense]
	php56-mbstring: 5.6.20 -> 5.6.21 [pfSense]
	php56-ldap: 5.6.20 -> 5.6.21 [pfSense]
	php56-json: 5.6.20 -> 5.6.21 [pfSense]
	php56-hash: 5.6.20 -> 5.6.21 [pfSense]
	php56-gettext: 5.6.20 -> 5.6.21 [pfSense]
	php56-filter: 5.6.20 -> 5.6.21 [pfSense]
	php56-dom: 5.6.20 -> 5.6.21 [pfSense]
	php56-curl: 5.6.20 -> 5.6.21 [pfSense]
	php56-ctype: 5.6.20 -> 5.6.21 [pfSense]
	php56-bz2: 5.6.20 -> 5.6.21 [pfSense]
	php56-bcmath: 5.6.20 -> 5.6.21 [pfSense]
	php56: 5.6.20 -> 5.6.21 [pfSense]
	pfSense-rc: 2.3 -> 2.3.1 [pfSense-core]
	pfSense-kernel-pfSense_wrap: 2.3 -> 2.3.1 [pfSense-core]
	pfSense-default-config-serial: 2.3 -> 2.3.1 [pfSense-core]
	pfSense-base-nanobsd: 2.3 -> 2.3.1 [pfSense-core]
	pfSense-Status_Monitoring: 1.0_1 -> 1.3_1 [pfSense]
	pfSense: 2.3_1 -> 2.3.1 [pfSense]
	perl5: 5.20.3_8 -> 5.20.3_12 [pfSense]
	pecl-zmq: 1.1.3 -> 1.1.3_1 [pfSense]
	openvpn: 2.3.9 -> 2.3.11 [pfSense]
	nettle: 2.7.1 -> 3.2 [pfSense]
	libzmq4: 4.1.3 -> 4.1.4_1 [pfSense]
	libsodium: 1.0.3 -> 1.0.8 [pfSense]
	libedit: 3.1.20150325_1 -> 3.1.20150325_2 [pfSense]
	gmp: 5.1.3_2 -> 5.1.3_3 [pfSense]
	glib: 2.44.1_3 -> 2.46.2 [pfSense]
	gettext-runtime: 0.19.6 -> 0.19.7 [pfSense]
	filterlog: 0.1_1 -> 0.1_2 [pfSense]
	dnsmasq-devel: 2.76.0test8 -> 2.76.0test12 [pfSense]
	curl: 7.47.0 -> 7.48.0_1 [pfSense]
	cpdup: 1.17_2 -> 1.18 [pfSense]
	bsnmp-ucd: 0.4.1 -> 0.4.2 [pfSense]

Installed packages to be REINSTALLED:
	rrdtool-1.5.5_1 [pfSense] (options changed)
	nginx-1.8.1,2 [pfSense] (options changed)

72 MiB to be downloaded.
Fetching unbound-1.5.8.txz: .......... done
Fetching sqlite3-3.11.1.txz: .......... done
Fetching smartmontools-6.4_2.txz: .......... done
Fetching rrdtool-1.5.5_1.txz: .......... done
Fetching rate-0.9_1.txz: ..... done
Fetching python27-2.7.11_2.txz: .......... done
Fetching php56-zlib-5.6.21.txz: .. done
Fetching php56-xmlwriter-5.6.21.txz: .. done
Fetching php56-xmlreader-5.6.21.txz: .. done
Fetching php56-xml-5.6.21.txz: .. done
Fetching php56-tokenizer-5.6.21.txz: . done
Fetching php56-sysvshm-5.6.21.txz: . done
Fetching php56-sysvsem-5.6.21.txz: . done
Fetching php56-sysvmsg-5.6.21.txz: . done
Fetching php56-sqlite3-5.6.21.txz: .. done
Fetching php56-sockets-5.6.21.txz: .... done
Fetching php56-simplexml-5.6.21.txz: ... done
Fetching php56-shmop-5.6.21.txz: . done
Fetching php56-session-5.6.21.txz: ... done
Fetching php56-readline-5.6.21.txz: .. done
Fetching php56-posix-5.6.21.txz: . done
Fetching php56-pdo_sqlite-5.6.21.txz: .. done
Fetching php56-pdo-5.6.21.txz: ..... done
Fetching php56-pcntl-5.6.21.txz: .. done
Fetching php56-openssl-5.6.21.txz: ..... done
Fetching php56-opcache-5.6.21.txz: ...... done
Fetching php56-mcrypt-5.6.21.txz: .. done
Fetching php56-mbstring-5.6.21.txz: .......... done
Fetching php56-ldap-5.6.21.txz: .. done
Fetching php56-json-5.6.21.txz: .. done
Fetching php56-hash-5.6.21.txz: .......... done
Fetching php56-gettext-5.6.21.txz: . done
Fetching php56-filter-5.6.21.txz: .. done
Fetching php56-dom-5.6.21.txz: ..... done
Fetching php56-curl-5.6.21.txz: ... done
Fetching php56-ctype-5.6.21.txz: . done
Fetching php56-bz2-5.6.21.txz: .. done
Fetching php56-bcmath-5.6.21.txz: .. done
Fetching php56-5.6.21.txz: .......... done
Fetching pfSense-rc-2.3.1.txz: . done
Fetching pfSense-kernel-pfSense_wrap-2.3.1.txz: .......... done
Fetching pfSense-default-config-serial-2.3.1.txz: . done
Fetching pfSense-base-nanobsd-2.3.1.txz: .......... done
Fetching pfSense-Status_Monitoring-1.3_1.txz: .. done
Fetching pfSense-2.3.1.txz: . done
Fetching perl5-5.20.3_12.txz: .......... done
Fetching pecl-zmq-1.1.3_1.txz: ... done
Fetching openvpn-2.3.11.txz: .......... done
Fetching nginx-1.8.1,2.txz: .......... done
Fetching nettle-3.2.txz: .......... done
Fetching libzmq4-4.1.4_1.txz: .......... done
Fetching libsodium-1.0.8.txz: .......... done
Fetching libedit-3.1.20150325_2.txz: .......... done
Fetching gmp-5.1.3_3.txz: .......... done
Fetching glib-2.46.2.txz: .......... done
Fetching gettext-runtime-0.19.7.txz: .......... done
Fetching filterlog-0.1_2.txz: . done
Fetching dnsmasq-devel-2.76.0test12.txz: .......... done
Fetching curl-7.48.0_1.txz: .......... done
Fetching cpdup-1.18.txz: ... done
Fetching bsnmp-ucd-0.4.2.txz: .. done
Fetching uclcmd-0.1.txz: .. done
Fetching libucl-0.8.0.txz: .......... done
Checking integrity... done (0 conflicting)
>>> Upgrading pfSense kernel... 
Updating pfSense-core repository catalogue...
pfSense-core repository is up-to-date.
Updating pfSense repository catalogue...
pfSense repository is up-to-date.
All repositories are up-to-date.
Checking integrity... done (0 conflicting)
The following 2 package(s) will be affected (of 0 checked):

Installed packages to be UPGRADED:
	pfSense-kernel-pfSense_wrap: 2.3 -> 2.3.1 [pfSense-core]
	pfSense-rc: 2.3 -> 2.3.1 [pfSense-core]
[1/2] Upgrading pfSense-rc from 2.3 to 2.3.1...
[1/2] Extracting pfSense-rc-2.3.1: .... done
[2/2] Upgrading pfSense-kernel-pfSense_wrap from 2.3 to 2.3.1...
===> Keeping a copy of current kernel in /boot/kernel.old
[2/2] Extracting pfSense-kernel-pfSense_wrap-2.3.1: .......... done
>>> Upgrading necessary packages... 
Updating pfSense-core repository catalogue...
pfSense-core repository is up-to-date.
Updating pfSense repository catalogue...
pfSense repository is up-to-date.
All repositories are up-to-date.
Checking for upgrades (59 candidates): .......... done
Processing candidates (59 candidates): .......... done
Checking integrity... done (0 conflicting)
The following 61 package(s) will be affected (of 0 checked):

New packages to be INSTALLED:
	uclcmd: 0.1 [pfSense]
	libucl: 0.8.0 [pfSense]

Installed packages to be UPGRADED:
	unbound: 1.5.5 -> 1.5.8 [pfSense]
	sqlite3: 3.9.2 -> 3.11.1 [pfSense]
	smartmontools: 6.4_1 -> 6.4_2 [pfSense]
	rate: 0.9 -> 0.9_1 [pfSense]
	python27: 2.7.11_1 -> 2.7.11_2 [pfSense]
	php56-zlib: 5.6.20 -> 5.6.21 [pfSense]
	php56-xmlwriter: 5.6.20 -> 5.6.21 [pfSense]
	php56-xmlreader: 5.6.20 -> 5.6.21 [pfSense]
	php56-xml: 5.6.20 -> 5.6.21 [pfSense]
	php56-tokenizer: 5.6.20 -> 5.6.21 [pfSense]
	php56-sysvshm: 5.6.20 -> 5.6.21 [pfSense]
	php56-sysvsem: 5.6.20 -> 5.6.21 [pfSense]
	php56-sysvmsg: 5.6.20 -> 5.6.21 [pfSense]
	php56-sqlite3: 5.6.20 -> 5.6.21 [pfSense]
	php56-sockets: 5.6.20 -> 5.6.21 [pfSense]
	php56-simplexml: 5.6.20 -> 5.6.21 [pfSense]
	php56-shmop: 5.6.20 -> 5.6.21 [pfSense]
	php56-session: 5.6.20 -> 5.6.21 [pfSense]
	php56-readline: 5.6.20 -> 5.6.21 [pfSense]
	php56-posix: 5.6.20 -> 5.6.21 [pfSense]
	php56-pdo_sqlite: 5.6.20 -> 5.6.21 [pfSense]
	php56-pdo: 5.6.20 -> 5.6.21 [pfSense]
	php56-pcntl: 5.6.20 -> 5.6.21 [pfSense]
	php56-openssl: 5.6.20 -> 5.6.21 [pfSense]
	php56-opcache: 5.6.20 -> 5.6.21 [pfSense]
	php56-mcrypt: 5.6.20 -> 5.6.21 [pfSense]
	php56-mbstring: 5.6.20 -> 5.6.21 [pfSense]
	php56-ldap: 5.6.20 -> 5.6.21 [pfSense]
	php56-json: 5.6.20 -> 5.6.21 [pfSense]
	php56-hash: 5.6.20 -> 5.6.21 [pfSense]
	php56-gettext: 5.6.20 -> 5.6.21 [pfSense]
	php56-filter: 5.6.20 -> 5.6.21 [pfSense]
	php56-dom: 5.6.20 -> 5.6.21 [pfSense]
	php56-curl: 5.6.20 -> 5.6.21 [pfSense]
	php56-ctype: 5.6.20 -> 5.6.21 [pfSense]
	php56-bz2: 5.6.20 -> 5.6.21 [pfSense]
	php56-bcmath: 5.6.20 -> 5.6.21 [pfSense]
	php56: 5.6.20 -> 5.6.21 [pfSense]
	pfSense-default-config-serial: 2.3 -> 2.3.1 [pfSense-core]
	pfSense-base-nanobsd: 2.3 -> 2.3.1 [pfSense-core]
	pfSense-Status_Monitoring: 1.0_1 -> 1.3_1 [pfSense]
	pfSense: 2.3_1 -> 2.3.1 [pfSense]
	perl5: 5.20.3_8 -> 5.20.3_12 [pfSense]
	pecl-zmq: 1.1.3 -> 1.1.3_1 [pfSense]
	openvpn: 2.3.9 -> 2.3.11 [pfSense]
	nettle: 2.7.1 -> 3.2 [pfSense]
	libzmq4: 4.1.3 -> 4.1.4_1 [pfSense]
	libsodium: 1.0.3 -> 1.0.8 [pfSense]
	libedit: 3.1.20150325_1 -> 3.1.20150325_2 [pfSense]
	gmp: 5.1.3_2 -> 5.1.3_3 [pfSense]
	glib: 2.44.1_3 -> 2.46.2 [pfSense]
	gettext-runtime: 0.19.6 -> 0.19.7 [pfSense]
	filterlog: 0.1_1 -> 0.1_2 [pfSense]
	dnsmasq-devel: 2.76.0test8 -> 2.76.0test12 [pfSense]
	curl: 7.47.0 -> 7.48.0_1 [pfSense]
	cpdup: 1.17_2 -> 1.18 [pfSense]
	bsnmp-ucd: 0.4.1 -> 0.4.2 [pfSense]

Installed packages to be REINSTALLED:
	rrdtool-1.5.5_1 [pfSense] (options changed)
	nginx-1.8.1,2 [pfSense] (options changed)
[1/61] Upgrading gettext-runtime from 0.19.6 to 0.19.7...
[1/61] Extracting gettext-runtime-0.19.7: .......... done
[2/61] Upgrading python27 from 2.7.11_1 to 2.7.11_2...
[2/61] Extracting python27-2.7.11_2: .......... done
[3/61] Upgrading perl5 from 5.20.3_8 to 5.20.3_12...
[3/61] Extracting perl5-5.20.3_12: .......... done
makewhatis: not found
makewhatis: not found
pkg: POST-INSTALL script failed
[4/61] Upgrading glib from 2.44.1_3 to 2.46.2...
[4/61] Extracting glib-2.46.2: ........

At one point ( probably during the glib extraction as it's the last line that was displayed ) :

• the web interface stopped responding ( eventually leading to a 504 timeout message )
• the Internet trafic routing was stopped
• the DNS queries were refused
• the only thing that was working was the LAN interface ping, and SSH

:(

I waited one hour, just to make sure it wasn't related to the upgrade itself, but since the appliance was still in the dark after that, I decided to reboot it  :-\

It's now running fine and says to be running 2.3.1, but :

• how can I check what went wrong during the upgrade procedure ?
• how can I be sure that the system's integrity is fine ?

???

Thanks beforehand  8)