Iphone 4 (ios 4.1 8B117) firmware 02.10.04

Hi Gurus, Is there a way I can go from ios 4.1 to newest apple ios without upgrading my firmware? I am using GEVEY SIM method to make calls on this SOFTBANK locked phone but now a lot of apps are not supported on this ios 4.1 and hence looking for solutions to upgrade ios while retaining my firmware. Gurus, Is there a solution out there that helps me? Thanks Anand