Ok guys I know I might sound like a noob or even an Idiot but How do I go from CFW to OFW. And then Can I go back???

I'm wanting to go and buy FTB3 but the problem is I Want to play online so I'm going to have to go to OFW. Yet I want to be able to keep playing ISO games with my little brother (He has an unhacked PSP fat with OFW 6.20 and I don't wanna have to buy more damn UMDs for no reason)

So can I go from CFW 5.50gend3full to 6.20OFW and then directly go back to CFW???