I have called ATT several times about the grandfathering and if you are upgrading like I am, they definitely will move your current plan to the new phone with a new contract whether you get it from Amazon or anywhere else. You just buy the phone and then contact customer support to activate and they simultaneously grandfather to the new phone.
As to the comment by nkd, you seem to forget that the Skyrocket (GS2) is the predecessor to the GS3 and therefore is indeed in the same league, not to mention it's been out far longer and therefore, far more stable. We all know that new devices will have plenty of issues, and am sure once they are shipping the GS3, plenty will show up. All you have to do is look at how many issues have surfaced with the One X and it's a good example.