Here are some discussions which suggest it’s not that easy:
https://news.ycombinator.com/item?id=28180590
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=976811
Also here is a list of changes which are not downwards compatible.
https://www.php.net/manual/en/migration80.incompatible.php
If you want to change, make a backup of your system and just try it 
edit:
okay in the nextcloud docs they recommend php 8.1
https://docs.nextcloud.com/server/latest/admin_manual/installation/system_requirements.html