Transfer Postpaid

I recently purchased an 8gig to replace my 4gig which I gave to my sister. When I activated the 4gig I used the 9999 trick to use a prepaid account. Now when I try to activate my new iphone it won't let me transfer the prepaid. Is there a way to activate it and keep my old number as well as a prepaid account? The only solution I can think of is porting my prepaid number to a tmobile prepaid and then porting that "t-mobile" back into a new activation of a prepaid on the 8gig. Any other suggestions would help. Thanks