Can I use the iPhone 5 I bought from T-Mobile for AT&T?

So, this is what's happening. I'll make it short and simple.
I'm planning on buying an iPhone 5 from T-Mobile right now. Problem? I'm already more than a year into my contract with T-Mobile, and I would like to move to AT&T by December (when my contract ends).

My question is: Will I be able to keep using that same iPhone 5 WHEN I DO move to AT&T?
Or will there be conflicts?

No not unless you get it unlocked.

http://www.powerglobaldesigns.com/iphone-without-contract

You can just get the iPhone 5 unlocked and use on AT&T.

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