The main reason to buy another carrier is to get more frequencies.
So I suspect that all past at&t phones and all past t-mobile phones will continue to work on this new at&t network the same as always.
What they WON'T do is gain much new coverage. But going forward most at&t phones will probably be tuned for more frequencies, meaning that future at&t phones will get wider coverage than current ones do.
But that's just a guess. The other thing they could do is turn all T-Mobile frequencies to LTE 4G and, yes, tell you all to get new 3G phones (for the old at&t 3G network). That's another option.