IPhone 3GS 4.1 won't allow manual upgrade to 6.1.2

Hello, I want to manually upgrade my jailbroken iPhone 3GS iOS 4.1 to 6.1.2 to allow keeping the jailbreak and the unlock (main reason). I already looked for tutorials, and all of them simply say to press "alt" while clicking on "upgrade" in iTunes. I select the IPSW for 6.1.2, and iTunes starts to update it, then fails, putting that "This iPhone couldn't be upgraded. This device isn't eligible for the requested build." I also try to put in in recovery mode, and this time, it fails with an error 1604 (related to USB timing). Why does this supposedly easy operation fails, and how do I upgrade to 6.1.2, but not 6.1.3? Regards