re: AT&T unlimited data grandfathered in

Switching phones won't change your plan. There's no mystical tech requirements in putting a sim card in a new phone that necessitate revisions in your contract, in the same way you'd upgrade from iphone to iphone. However, AT&T representatives do make commission from certain things, and I wouldn't be surprised if they are incentivized for moving people over to new plans with data caps. As long as you read anything you sign carefully and make it clear you want to keep your existing plan 100% intact, there shouldn't be an issue. They go after the low-hanging fruit, people who didn't even know they were on the unlimited plan, have no idea how much data they use per month, and are sold on the price difference (in the plans themselves).

For the past several years, I've been doing phone upgrades at Radio Shack of all places. They love to pressure you to get cases, screen protectors, extended warranties, etc., but one thing they don't do is try to frick with your existing plan because they likely have nothing to gain from it.