Change my sim from T-mobile (pre paid) to AT&T (pre paid)?

i have a T-mobile SIM card i want to change it to an AT%26amp;T while retaining my t-mobile number. where can i do this? and how? i live in los angeles Change my sim from T-mobile (pre paid) to AT%26amp;T (pre paid)?
Well to my knowledge, the only way to keep your number would be to go to an ATT store and ask them to keep your number, when you get a prepaid ATT sims card and you call to activate they just give you a number so I don't think it would be possible when your activating your new sims over the phone.