Update your system sources and read /usr/src/UPDATING ;-)
Shouldn't be much trouble other then a lot more mergemaster work in /etc/ I think.
Use the kernel configuration in /PCBSD/conf/ that relates to your system (e.g. PCBSDv1.3 or PCBSDv1.3-SMP e.t.c.) instead of the GENERIC config in the FreeBSD sources. Other wise you can break a few things lol.
If you update the kernel but not the rest of the FreeBSD base they say you can break a few things. So remember the handbook well xD