iPhone 3GS Upgrade/Jailbreak/Unlock Help
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&amp;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&amp;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