I just played around upgrading a FreeBSD 14.3p6 VM to 15.0-RELEASE via FreeBSD-base.
I still have not figured out how to do that with the official repos, but after building packages locally i was able to upgrade the boot environment and reboot in a working #freebsd15 installation!
Has anyone got this working with the official base repo?
If anyone is interested, here are my notes about upgrading locally (Notes in picture alt for copy paste).