AT&T does not lock anyone into 2 year contracts any more that T-Mobile does. Both of them offer month to month (on any phone) and both of them offer discounts on new phones if you get a 2 year agreement.
If you bring your own (compatible) device, you can get a no-contract plan. The prices are exactly the same. If you want a no-contract plan, but don't have a device, you have to pay the un-subsidized price for the phone.
I've never had a problem swapping SIM cards on ATT. Not sure if it's "supported", but I've done it in the past. The only phone where I've run in to restrictions was the iPhone. They ask you for the IMEI because they show "your device" in the ATT portal. I don't think it's used to restrict your access.
Not trying to make an argument out of it or anything :) but I figured since you're a T-Mobile customer, you're already in for a rough ride :( Thought I'd try to offer some consolation as we welcome you to the support group... I mean, network. Yeah, network.