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Okay, so moral agreements aside...

YES. Swapping the SIM card WILL WORK.

AT&T cannot go and change something on your contract (like the data plan) without your permission as it violates your cell phone contract.

If, by some chance, the data doesn't work, go to http://help.benm.at/ on Wifi and download the ATT Carrier File, and it will start working (this was required for the $15 data plan to work on my phone).
 
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