It's very simple actually.
Make sure that while she is still using her current Blackjack that you get ATT to disable all network connectivity, even the pay as you go data access junk.
Jailbreak and Unlock your old iPhone, put her SIM card in there, and you are ready to go.
While ATT would like to make you think that they can add the data package to your plan, they physically cannot if your IMEI numbers do not match, which they wont once you swap the phones.
Fairly simple, right?