If you want to make kernel-smp-devel and kernel-hugemem-devel be install vice update, then add them to a line like this:
installonlypackages=kernel-smp-devel kernel-hugemem-devel
that should make it work the way you want.
Thanks, this wasn't so much an issue with it being upgrade instead of install but just seemed like an inconsistency - there's conceptually no difference between kernel-devel and kernel-smp-devel and they were being handled differently...
Cheers, MaZe.