What makes you think you can't update your iOS?

Granted, I left the iPhone camp about a year ago and havn't really looked back, but the normal deal was that if you have a JB, you can upgrade to a later version and keep the JB, as long as you never install a stock update.

I always used pwnage tool and that worked well for me. Your milage may vary, but a quick google search should give you all the facts.