When upgrading to the most current UBB.threads version available, be sure you are using the most recent version UBB.threads archive. It is very rare but it does happen, that only the installer/upgrade scripts
within archive sometimes get revised to fix rare/fringe install/upgrade issues as they are discovered.
For example, on March 31st 2020, the installer/upgrader scripts
within UBBT 7.7.4 received fixes related to UBBT 7.0.0 (2006: cache builder), MySQL 4 (2008: storage "type" vs "engine"), language strings from UBBT 7.0.0 (2006: duplicated entry terms), and a conditional statement correction related to php 7.1+ for disabled styles.
If you you're already running the current version of the forum software, there is nothing within the installer/upgrader that affects you or changes your already installed software.
As posted earlier in this thread by Ruben, this is a correct path for upgrading from UBB.threads 6.5.5 (released May 31, 2006):
upgrade to ubbthreads 6.5.5 then go to say ubbthreads 7.2
After that you should be able to upgrade to the current v7 version.
Just read the upgrade directions in the install folder at each step.