Just for reference purposes, I just updated a server to php71, and Joomla and Wordpress run fine on it. The speed of loading is dramatically improved. Fortunately, UBBThreads is on a different server, so that can wait until the work is done.
Server Information UBB.threads Version 7.7.5 Release 20201027 Server OS Linux Server Load 0.16 Web Server Apache/2.4.6 PHP Version 7.4.33 MYSQL Version 5.7.43 Database Size 1.85 GB