This just in from Droid Life, apparently the new .621 breaks the ability to use older firmware files in RSD Lite, meaning, you will NOT be able to return to .605/.602 or .340, at all.
If it breaks this, I would assume the bootloader has been updated (unlocked? Doubtful...) and I would also assume it breaks root as well!
If you make an attempt at an SBF to an older android version, you will have a very nice PAPER WEIGHT! AKA BRICK!
Please take caution when taking the update, if you do take the update, don't dinc with it!
You have been warned!
DROID X Update To .621 Breaks Ability To SBF Back To Earlier Firmware – Droid Life
While there is no way to go back to previous versions, a 621 SBF has been leaked over at XDA
[SBF and Update] Original Gingergread 2.3.4 SBF for Droid X MB810 Verizon USA - xda-developers