Gophone plan and device change; possible?

Currently I have a basic phone and 10 cents per minute plan.  I want to switch to the Gophone $25 monthly 1.5G data only plan to use with the Mifi Liberate hotspot that I bought recently.  I want to move the phone number to the new device in order to keep my current account credit.  It also looks like I'll need to get a new SIM for the hotspot as I can't seem to be able to take the phone apart to remove the current SIM.  Can I make this kind of switch with Gophone?  Can I do this in a store or do I have to do it online or by phone?  Ultimately I want to go to a postpaid data plan, but I want to trial it first using my current Gophone status to make sure I can get service in the locations I need to have it before commiting to postpaid.  Thus, there is the possibility that I'll find out AT&T can't provide service where I need it and may want to switch back to my basic phone, keeping the same number.  If that's the case will I be able to revert back?