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 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).

Create Boot Env and mount it
```
bectl create 15.0
mkdir /mnt/upgrade
bectl mount 15.0 /mnt/upgrade
```
Change pkg configuration `/mnt/upgrade/usr/local/etc/pkg/repos/FreeBSD-base.conf` to local repo
```
FreeBSD-base: {
  url: "http://local.ip/${ABI}/15.0",
  enabled: yes
}
```
and comment out FreeBSD-base from /mnt/upgrade/etc/pkg/FreeBSD.conf
Install FreeBSD-pkg-bootstrap, as mentioned in the [release notes](https://www.freebsd.org/releases/15.0R/relnotes/)
```
env ABI=FreeBSD:15:amd64 IGNORE_OSVERSION=yes pkg -c /mnt/upgrade/ install -r FreeBSD-base FreeBSD-pkg-bootstrap
```
Then upgrade the base packages
```
env ABI=FreeBSD:15:amd64 IGNORE_OSVERSION=yes pkg -c /mnt/upgrade/ upgrade -r FreeBSD-base
```
Check, if FreeBSD-set-base is installed:
```
env ABI=FreeBSD:15:amd64 IGNORE_OSVERSION=yes pkg -c /mnt/upgrade/ install -r FreeBSD-base FreeBSD-set-base
```

upgrade all other packages
```
env ABI=FreeBSD:15:amd64 IGNORE_OSVERSION=yes pkg -c /mnt/upgrade/ upgrade
```

Temporarily activate boot env and reboot
```
bectl activate -t 15.0
shutdown -r now
```
0

If you have a fediverse account, you can quote this note from your own instance. Search https://mastodon.bsd.cafe/users/lukas/statuses/115674152709612164 on your instance and quote it. (Note that quoting is not supported in Mastodon.)