Just get the closest release. Usually not having an exact match is fine.
And you can also overwrite both base and the kernel, that's effectively the same as a reinstall except that you don't lose all the stuff outside base (e.g. /etc, /var, /usr/local, etc.)
In dem olden days the sets came with an install script, but no longer seems to be the case. You probably want to make sure that:
- base.txz doesn't overwrite stuff you don't want it to overwrite (like stuff in /etc. IIRC it doesn't, I haven't used FreeBSD in two years so perhaps this changed).
- Use the correct flags to tar to preserve permissions, flags, and such.
You may also be able to use my
pkg_sanity script to clean up some of the mess, by the way.