as a current dev of ubbthreads, i will switch between php 7.1 and newer versions constantly, to test different features for any compatibility issues.
i also use bluehost to host a test forum and several of my live forums.
switching php version can be done on the fly with bluehost. no need to close your forum at all.
visit the multi-php option in your bluehost control panel and select to use their default php version, or anything from php 7.1 and newer.
the current public version of ubbthreads is 7.7.5 and does not support php 8. the developer version of ubbthreads (not yet released) supports php 8, and will be available for download soon in the ubbcentral members area.