Postpaid phone service to Prepaid phone with postpaid service

About 4 months ago my LG G3 broke and I needed a new phone so I purchased the Galaxy Note 7, but soon after that the phone had to be recalled. I wanted to wait until the new Samsung phone came out but I realized it was too far away, so I really needed a good cheap new phone that I could use in the mean time. I was told by AT&T customer service that I would be able to stay with AT&T and use the postpaid plan that I had with a GoPhone. So after that I purchased a Galaxy Express Prime as a GoPhone on the AT&T website. After receiveing the phone I realized that my old SIM card was too big for the Prime. What should be my next course of action to keep my old number and my old plan and use it with the GoPhone. Please HELP!!!