I have two iPhones, A & B

I activated iPhone A through iTunes, selecting current AT&T customer and adding the $20/mo internet.

I unlocked and jailbroke iPhone B and was using T-Mobile.

Now iPhone B is not working and I want to take it in to Apple under warranty.

I have virginized iPhone B but I want to take it in using the SIM card from iPhone A (so it appears as a legal iPhone with a problem)

But when I put the SIM card from iPhone A into iPhone B and connect to iTunes, iTunes wants to activate it.

Do I continue with the activation through iTunes? Using all of the info from the account from iPhone A? Will this mess anything up with iPhone when I try to put it's own SIM card back in it?

Does Apple/ATT know which SIM cards were originally activated with each iPhone?

If you have a problem with your legal iPhone and have to do a restore, do you have to go through the activation process again?

Thanks