@Stewart @SteveITS
This was solved here.
Due to changes in pkg, the new version of pkg may not be able to properly locate and use the CA trust store when running on the previous version before upgrading.
If the firewall is unable to load packages or check for updates after selecting the CE 2.7.1 upgrade branch, run certctl rehash from the console, a root shell prompt, or via Diagnostics > Command Prompt. This will allow pkg to utilize the system certificates until the next reboot.
This is true in 2.7.0 as well. Running the certctl rehash brought it up to be able to upgrade.