App Store compatibility

I have a gen 1 iPhone running 1.1.4. I used Zibri (which I have now been schooled that this is not the best?) to unlock and jailbreak. I don't really care about getting 3rd party apps but I wish to continue running my phone on t-mobile and want to get into iTunes app store and take advantage of the many apps.

My question...is there a way to upgrade to a newer software version so I can get in app store and still keep the phone unlocked? If this is possible could someone please provide instructions? Thank you for any help!