What is involved with buying a new, unlocked smartphone?

I use AT&T. I have had 2 Samsung smartphones. I currently have a Galaxy S5.

I don’t mind the Samsung interface they build on top of Android, but I hate the fact that I get Android upgrades a year after they are released. Upgrading from Android 4.4 to 5.0 was a pain, too.

While I say I don’t mind the Samsung interface, I detest bloat. The phone comes full of Samsung and AT&T apps that I have honestly never once opened. They just sit there taking up memory space.

The S5 I have currently is 2 years old. It’s running fine. No major complaints. I don’t need a new phone today.

I am seriously looking at a Nexus phone. I am fully enmeshed in the Google-verse and love it. (I will not consider any Apple products. Thanks.)

Google will launch a new operating system and new phones in the near future. I will wait to see what those phones have to offer.

I have never bought an unlocked phone before, and I want to know what I’m going to have to do to use it.

Do I have to take the phone into my AT&T store to get a new SIM card? My Samsung Galaxy S5 uses a micro SIM card, and the new Nexus phones require a nano SIM card.

Can I order the new nano SIM card online?

What is involved with buying an unlocked phone? How do I switch from my AT&T phone to an unlocked one using it on AT&T?