n2, your best bet would be to sbf to .602 or .605, then root and install root keeper, create a backup in recovery, disable update checker (so you don't get updated to .621), then restore just apps, no system data (no green, no red).
If you think you want to go to .621, you'll want to use root keeper. Reports have been good, but I don't know if it's 100% guaranteed to work. .621 can't be rooted, but there are work-arounds (that I'd rather not use).
Are you determined to stay on the stock rom?