Re: Recently involved in AT&T next plan. Looking to sell Note 3 and buy unlocked Nexus 5.
The answer to both your questions is yes. AT&T doesn't care what you do with the physical device, it's the line that's under contract. In case of Next it's you paying the device off they care about, you don't have to turn it back in. And both phones use a micro-SIM, so no problem swapping.
*The views and opinions expressed on this forum are purely my own. Any product claim, statistic, quote, or other representation about a product or service should be verified with the manufacturer, provider, or party.