Can I sell my iPhone 5s under contract AT&T?

I bought an unlocked iPhone 6, swapped the sim out of my old 5s. Everything is working fine I just want to know if I can sell the 5s without voiding my contract or affecting my plan? I tried to talk to a represenitive at ATT but they didn't quite understand what I was asking.

Seems to me that you should be able to -- otherwise, it would be called renting the 5s from AT&T, which it isn't

NOTHING you do VOIDS the contract. If anything you suggest VIOLATES the contract, it would be swapping the sim card to a different phone. Aside from possibly charging a fee, I see no reason why they would care.

You can do whatever you want with the phone as long as you continue paying the contract until done. 5S remains locked until contract is paid off.