Dont worry about it, the data charge will not bill on the upgrade line most likely, if you do the activation and the esn swap the same day, you shouldnt have to worry about it. Does your mom have TEC? I would recommend keeping that, if she still has it, i would recommend her trying to get a replacement first, there is a possibility she may get a newer model, you will see what happens.
Most likely you would get a CLNR, which is fine. I had my touch replaced on one of my lines and the CLNR is working better than the original, so im happy with the replacement stuff. And its nice to know i could always buy a cpo if i wanted.