Yes, I got to keep my unlimited data and my old "iPhone Messages 200" plan (I hardly ever text anyone who doesn't have an iOS device). This was explicitly stated on my order confirmation. I placed the order through the Apple Store app on my phone. Got a 6+ 64GB. Not arriving until 10/1 - 10/6, though. :/
I was at the tail end of a 2-year contract with AT&T. I think my official contract expiration date is 9/22, but they've been showing me as eligible for a full subsidized upgrade since last week at least.
Customer since 2/2008.