PS3 Slim 320GB CECH-2504A CFW to OFW and back

Hello all!
I have an PS3 Slim 320GB CECH-2504A with Rebug 4.88 CFW. I want to give this console to my friend who has a plenty of games on disc, and on PSN. He want to play PS3 games and get trophies on his regular PSN account used on PS5 and PS Vita too.
The question is how to go to OFW (I think 4.91 is the latest), and after he played those games (a few months) on his account, how to go back to Rebug/Evilnat CFW?

Thanks!