first make a snapshot of your current vm so you can go back to it if things go boom. i would highly advise to make a backup using the virt built in backup system. erase the vm…install the new version minimal from scratch…install virt with your installer…then pull the backup into the system…reduces the chances of issues.
Maybe to get php 8.1 without adding extra ppa’s. If I remember correctly php 7.4 comes to end of life later this year, or save going for 22.04 you could do this
I went down the ppa route to add updated libs & PHP to date this is working fine but there will be a point in the future where everything fails ! I am doing some ground work before I do what you want to do I’ll let you know how it fairs