How would returning an iPhone X work?

I've had all iPhones from the 3G on and never had to return one.
I am not sure I am going to like the new X coming from the 7Plus but I want to still order the X when they become available and try it.
My question is:
If I decide I don't like it and want to return it, how would that work exactly?
I would be tied to the AT&T next plan and still have my old phone (7 Plus) available to go back to.
Would I need to take both phones in? And would I take them to Apple or AT&T?
Who has actually done this before?
TIA

 

I thought they had different size sims but I'm not sure.

 

Thank you!