saving balance on a go phone account after transitioning to post-paid

Hi everyone.

I have a prepaid go phone with ~$100 on it. Recently I bought a smartphone (paid in full no contract) and tried to port the number to save the balance from my go phone account, but was told at the store that it is not possible. I opened a new account online, and got a new number, but I am wondering if there is anything can be done except to let the entire go phone account balance expire?

I called AT&T, both prepaid and post-paid. A representative escalated the case to the supervisor, and a supervisor wrote an escalated request to transfer the balance over. The request has been denied, as I am being told. Customer service tells me to go to the store, cancel the existing post-paid line, port the number and pay new activation fee. At the store I am being told that it is not possible to port that go phone number.

Is there anything I can do to save the balance? I was thinking of selling the sim card on ebay or craigslist, but the balance expires on 05/09.

Thanks a lot for help.