After updating to 3.55.2 Rebug i was NOT able to simply jump back to a older fw version. Even with system debug option. There for we still would need the special downgrader pup's to be able to downgrade software wise from xmb with this new function. But on Rebug you still can use togle QA (if you have untogled your con after) and just downgrade back to the fw version you want.
Maybe we will find a way to convert the special downgrader pup's to use on retail cons. Should be possible