I have a jailbroken, unlocked iPhone 3GS that i would like to upgrade from 3.1.3 to 4+ without messing up my unlock. Here's the phone specs:
- Baseband: 05.12.01
- SHSH blobs in Cydia: 4.1, 4.2.1r1, 4.3.3
- T-Mobile prepaid SIM (in US)
I've backed everything up on iTunes and using Tiny Umbrella. I've read quite a bit about the process online, and as I understand it I need to create a custom IPSW to preserve my baseband and thus the unlock (I'd like to avoid going the GPS-breaking iPad iOS route if possible). However, it appears the only way to make a custom IPSW is with Pwnage Tool on the Mac, and I have a PC.
So...I found this link online that includes detailed upgrade instructions as well as links to a pre-made custom IPSW. My questions to you: does this process look legit? Should I download a premade custom IPSW? Is there a more trustworthy source for a custom IPSW and/or instructions for upgrading my iOS?