IPhone 4 Jailbroken T-Mobile?

I recently got an iPhone 4 as a gift- already jailbroken ios 4.1.1. I had to cut my T-Mobile SIM from my previous Sidekick 4g into a microSIM (yes, with scissors) and I actually get service.

However, I'm getting pretty tired of having no apps I can use that require ios 5 or more and was thinking of upgrading to ios 6.1.3, which unfortunately, has no current jailbreak.

If I do upgrade it via iTunes, will I still retain my service from T-Mobile?

No upgrade your unlock is gone there are unlocks for 6.1.3 but i never tryed any

Yes.