Just to clarify for those who may not be familiar with the lingo.

When you fly AirTran, you generally get 1 credit per one way coach trip. Once you have enough credits, you convert 8 credits to a one way coach reward voucher. You can exchange this reward voucher for a one way coach reward ticket.

The OP is correct in that you can also convert 4 credits to a one way upgrade voucher, and you can exchange the upgrade voucher for an upgrade to business class on a one way flight (origin to destination). The ticket you are upgrading can either be a paid ticket, or a reward ticket. HOWEVER, in my experience, they will not let you upgrade a ticket acquired as a result of a bump voucher. I would be interested if anyone has other experience on this.