I have been a long time ATT customer as well, and if I remember correctly this is how it has gone with the iPhones....
Original iPhone was not subsidized, but you did have to sign a two year contract.
iPhone 3G everyone was eligible to upgrade to since there was no subsidy on the original iPhone. You just had to signs a new 2 year agreement.
iPhone 3GS everyone had to play by the rules of ATT upgrades which we are facing now. (The 2 years / 19 months/ the more you pay the sooner you can renew rules.)
iPhone 4 ATT let everyone upgrade because they knew their exclusivity was going to run out within the next year and wanted to get as many people locked in a possible.
iPhone 4S we are back to the regular upgrade rules again.
My bet is that People who bought the 4S won't be eligible for the iPhone 5 when it comes out.
They way I see it the rules have always been the same, there has just been two years where ATT made exceptions, (iPhone -> 3g, 3GS ->4).
Maybe I am odd...