I just updated a FreeBSD 14.3 jail to FreeBSD 15.0 using mkjail. This is unformatted. Scroll to the bottom for the actual update.
In short, use the FreeBSD pkg repo until you get your host/poudriere running.
https://dan.langille.org/2026/02/08/updating-pkg01-to-build-15-0-packages/
Not shown, the jail host was updated first.