SIM card swap, is it just that simple?

[TheRetiredSlave]

Totally noob questions here.. Sorry.

I currently have an iPhone 5s and a contract (+decent data plan, all that jazz) with AT&T, but I kinda want to try an HTC One M8. So my question is this, can I just buy a used One M8 on the cheaps and swap my current nano-SIM from my 5S to the M8, and everything will work fine?

Will it just automatically work with my current contract without any problems or is there something else I need to know (like needing to call ATT, changing an account setting, etc?)?

And if I don't like the M8 or need to use my iphone for something, will changing back be just as simple as swapping the SIM back?

(Bonus question, with the SIM card removed from the 5s, will it still function as a glorified iPod Touch or will it not function at all?)

[Kevinkt]

it should. when i got my phone the sellman was retarded and give me the wrong colour( i know world first problem) and i changed the sim and everything worked fine

[elecxonica]

If it's compatible with AT&T then it'll work perfectly.