I am not sure if you can or not, never heard of that. I am not sure a customer service agent would be able to do it or not. An easier and probably less time consuming option might be to try and contact some other fans going to the Miami shows with GA tickets as a first option. Doesn't hurt to call Ticketmaster, oh wait, actually most of the time it does...
My Miami GAs just came from TM last night. They say FLR and FLOOR/GENADM. Perhaps you have reason to worry. There was a thread where someone did contact TM to verify some aftermarket tix they bought, and they actually got a reply. They turned out to have a valid code number, but TM couldn't guarantee they weren't fakes/duplicates.