I did the upgrade for our trip last October. It must be a fairly simple process, because my dad did it for me over the phone

. When I realized that I would be at a lunch meeting (without internet access!) at the 24-hour mark, I called my dad and gave him my info. He was able to check us in and get the upgrade within about 5 minutes. He laughed when he saw our seat assignment since we moved from Row 22 to Row 1

!
You can find the price on the AirTran website. I believe you go under "travel tools" and click on "business class". It mentions a "limited time promotion" (not sure what "limited" means since it has been ongoing for over a year!) which allows you to upgrade for as little as $49 each way. There is a list of routes and what the cost is. The longer the flight, the higher the cost. We fly Dertroit to Orlando and it is $69 each way.
I am planning on doing it again for our trip in June as a surprise for my DH. (He did not go with us on our last trip) I loved the complimentary adult beverages. I got my money's worth on our flight down!

I don't bother with the upgrade on the return though since I have no "magical express" to make my 2 hour drive home!