I have a jailbroken iPhone 3GS running iOS 5.0.1 with baseband/bootrom 05.16.02. I want to upgrade to iOS 5.1.1 but also make my device unlockable. With that baseband, however, I can no longer unlock my phone. I know I can install baseband 06.15.00 for the iPad WiFi+3G that Apple bundles with iOS 3.2 to 3.2.2 and then unlock, but that breaks GPS functionality.
However, according to this post it looks like it's now possible to go back to baseband 05.13.04 from 06.15.00, which still allows for unlocking. But I'm SOOOO confused.
So I install baseband 06.15.00 using a custom ISPW for iOS 3.2 to 3.2.2, jailbreak and unlock with redsn0w, then downgrade baseband from 06.15.00 to 05.13.04 while upgrading iOS 5.0.1 to 5.1.1 all with another custom IPSW, and then re-jailbreak?? Ugh!! My head is spinning!
Best Answer
I'm unfamiliar baseband, but i've used this site to upgrade/downgrade iDevices in the past.. Simply download the iOS version for your model device, restart iPhone in recovery mode (hold home&power buttons while restarting until it shows the recovery mode screen). Then iTunes takes over for you to install the legit iOS version.
I'd recommend using this method to get your iPhone where you want it version wise, then use whatever jailbreak method you'd prefer to jailbreak it afterward.
Also, if you're simply trying to upgrade this phone to later sell it, then you don't need to jailbreak it to unlock it. AT&T (only one who had the 3GS in USA) said a year ago that if an iPhone is not currently in contract that all you have to do is take it in the store and they'll unlock it for you for free. I hope that helps a little