Using a post paid SIM card in an htc inspire to use it as a prepaid?

I'm sorry if that makes no sense. I have an htc inspire I've had for almost two years now, and have been an AT&T customer for almost four. My mom and I have always spilt the bill, but lately she has not been able to pay her half, and I can't pay the whole thing, so it got turned off. Could I buy a prepaid AT&T go phone and put my normal AT&T SIM card in it and use it as pre paid?? Basically, I just don't want to have to get a new phone number to get pre paid, and I want to stick with att so I don't see why that should be a problem. So please let me know if that is possible, and or if there is a way for me to keep my phone number. Thank you!!