In FIM RTM version, there was an issue when you had to go back and try to re-install something after you'e done an upgrade to Update1. The error was due to a newer version of DB that FIM RTM couldn't handle. Big headache.
If I decide not to install a feature in R2, like the password portal, to begin with, then upgrade to the next release - will I be able to go back and install a feature without huge headaches? of course then reapply any updates.