I'm not eligible for a phone upgrade to the iPhone but my mom's phone is?

Okay so the only thing i asked for this year was an iPhone. turns out i got my phone last year about 13 months ago and won't be eligible for a phone upgrade until may. if i dont wait until may, the iPhone costs 400 dollars. However my mom and dad's phones are both eligible. since i have cingular, couldnt my mom just get her phone upgraded to the iPhone and then we could switch SIM cards so she keeps her old phone and then i put my sim card into the iphone? do anyone think that cingular would know and get mad about this? i mean its not like im stealing from cingular...