AT&T has issued a press release today that indicates you’ll be able to keep your old grandfathered-in unlimited data plan on your new iPhone 5 even if you upgrade using its subsidized pricing, as pointed out by Matt Buchanan on Twitter.

AT&T’s Ralph de la Vega had this to say about the iPhone 5 and the upgrade process, emphasis ours:

We offer customers the flexibility to keep the iPhone data plans they already have or choose any of our individual or new Mobile Share plans. We’re proud that more customers choose AT&T for iPhone than any other U.S. carrier and look forward to making iPhone 5 the newest addition to our lineup.

So it looks like grandfathered plans are all good on AT&T, so that should please those of you who have yet to change your plans from way back in the day. Those plans, of course, are only really unlimited up to 5GB as AT&T will then throttle your 4G LTE speeds way down until the next billing cycle.

This is actually an upgrade in two ways. First, AT&T throttles at 5GB instead of 3GB, where it used to, and those unlimited data plan users will essentially get a free upgrade to LTE.

Verizon has taken a different tack, allowing you to keep old unlimited data plans, but refusing to subsidize their phones. That means that you’ll have to pay the full rate for your iPhone 5, which starts at $500 and up, if you want to keep your old plan.