Droid 2 update 4.5.621 breaks root, and blocks all known exploits, which means at the moment you cannot re-root after installing the update. However there is a way to retain root and still get the update.
If you update to 4.5.621 and want to retain root you must install RootKeeper first.
RootKeeper is an app available on Google Play. Simply install, open, and run a backup. Then after installing update 4.5.621 use RootKeeper to restore root permissions.
I can confirm it does work, and for right now at least is the only way to be rooted on Droid 2 system version 4.5.621.