The version of UBB.threads that's recommended would be "newest possible", things are updated and streamlined in PHP with new builds. UBB.threads v7.7.5 will run on PHP7+ but nothing currently works with PHP8. All builds of PHP7 should be supported.
- Is there a guide for changing the PHP version on an existing forum? If so, where would I find it?
This is a setting in your webserver settings, UBB.threads doesn't really give a crap what it runs on, so long as it's supported.
- Is the process as simple as closing the forum, selecting PHP 7.4 in cPanel, then re-opening the forum?
Yes, again UBB.threads just wants a fully functioning webserver.
- Is this a one-way trip, or can we fall back to PHP 5.6 to keep the forum running, correct any errors, then try again?
As it's a webserver setting, it doens't matter; if you were to upgrade UBB.threads then that upgrade cannot fall back (once a db update pushes through theres no reversing it). But this is all webserver changes.
No matter what happens, web server settings have to line up with requirements of UBB.threads; this is what the UBB.threads System Requirements Test
was developed for, it'll check some common webserver issues and is developed by myself completely independant of UBB.threads Development which is contracted through UBBDevelopers Network
which is a partnership of Isaac and myself.