1.2.3 i386 to 2.0 amd64 possible?
Upgrade via the web GUI, is it possible? A simple yes or no will suffice ;)
Upgrading from 1.2.3 to 2.0 amd64 might have issues, but I've heard of people upgrading from 2.0 i386 to 2.0 amd64 with some success. You may be able to upgrade to 2.0 i386 and then from that to 2.0 amd64. I think I've heard that the format of the RRD databases is a bit different between 2.0 i386 and 2.0 amd64, so you may need to clear out those files if you want the RRD graphs to work.
In theory it should work though the last time I tried it in a VM the upgrade locked up - though it did complete, I had to force a manual reboot.
We have some workarounds in place to help facilitate the switch (it keeps a copy of the 32-bit reboot binary aside just in case), but apparently there's more to it than that now.
You do have to completely delete all of your RRD graphs after the upgrade because the data format changes from 32 to 64 bit.