Why don’t you contact AT&T customer care? I find the most capable technicians who can help can also be in-store engineers (techs) at the AT&T service centers (not the independent sales franchises, but the actual brick-and-mortar AT&T stores). They have trained techs who can program the iPhone and should be able to answer your question.
In my experience, I had a warranty trade-in of my first iPhone, because of dead/stuck pixels on the screen; I took it in to an Apple Store, and they had to do some programming on the computer before swapping the SIM card out. But I do remember, they were able to transplant the SIM itself. Now, I’m not sure if there are different sizes of SIM cards (there may be), but in any case, they had to program the IMEI into the computer to update the SIM when they initialized my current (replacement) iPhone.
I don’t see why you can’t just ask them up-front. As for the jailbreak status, they don’t really care, except I’d recommend that you first restore the phone (be careful to follow the proper procedure that your jailbreak software dev. indicates for restoring the phone-they vary from software to software).
IAC, it can’t hurt to ask questions prior to taking action.