If you really want to downgrade, then either use hell cats recovery flasher or do it this way, install official firmware 5.55, then upgrade to 6.20 from 6.20 downgrade back to 5.50 OFW and then install CFW, caution risks are involved using this method. The downgrade is made davee i think i am not sure. But this method should be possible since you have one of the oldies. Still be warned risks are involved and i or this site will not be held responsible.
TrueBlue, You Lose.