There is no reason you can't keep your unlimited data plan, but keep an eye on your plan for a week or so after you do the upgrade. I didn't have a problem when I bought my GSII from Best Buy, but when I bought my Note from Costco I noticed they put me on a limited data plan even though the Costco person said my plan was unlimited and would stay that way. You can get it back, but it is more of a hassle than it use to be. The ATT rep that I talked to (seemed knowledgeable about ATT plans and phones) said it used to be that they could just switch people back. Then they were told they needed to get their supervisors OK. Then ATT made it so they had to submit a request into "the system". It took two days for them to text me that the switch back to unlimited was approved. He said if you catch it quick it isn't a problem, but some people don't notice for months and after that long ATT won't put them back.
It seems like the problem is Costco related as a friend recently got a GS3 from Costco and the exact same thing happened (had to submit request and got approved a few days later). Bottom line.... watch your plan like a hawk for a week or so after upgrading to make sure nothing goes wrong.