upgrade of poudriere builders to major versions

Hello,

I build in poudriere jails, and upgrade my hosts so that my jails have an
equal or inferior version.

I also build my poudriere builder machines inside poudriere (using the
poudriere image option, which replaced my nanoBSD setup).

What's the recommended way to jump from 14 to 15? I can't use my 14 builder
machines because the jail is not happy and the ABI changed enough in 15
that it doesn't work (IIRC 13->14 was ok).

How is FreeBSD's releng infrastructure deal with this?

Thanks!

YAnn