I currently have an att iphone 5 gophone prepaid plan and I want to switch over to my sister's att postpaid family plan. But I was told at the att store that this cannot be done because I cannot port my old number from att prepaid to att postpaid. I would have to get a new number (I want to keep my old number as I have had this number the last 10 years). Has anyone come across this issue and what can I do to switch to my sister's postpaid plan while keeping my old number? Thanks.