A while ago, I bought an iphone 3GS untethered and jaibroken, so I could use it with T-mobile. This was when at&t was the only carrier of the iphone, and t-mobile did not carry the iphone. I got t-mobile service on it and have used it since, but when I plug into itunes, I am prompted for an update to the iOS, from 4.3. (what I have now) to 6.0. I have always chose no update in itunes, for fear that I would lose the jailbrake, and lose my t-mobile phone service on it. if I updated the iOS, would I lose my current T-mobile phone service with its current plan?