Thank you all for your help. For once it sounds like hassle-free gig. I'm happy to have that 4" option. I was thinking when the 5c croaked I might have to go back to just using a dumb phone for calls and an iPad mini for data.
I thought I'd throw a few things out there since I've had AT&T service with a work discount, manage our work plan, and have helped a friend rework theirs. AT&T corporate store policy (maybe unofficially?) is that if you need a different SIM on an active line, they won't charge you. Activation charges tend to be only with a new line or if you're upgrading with the older contact methodS. If you have a discount through work/school/etc, it will stay on your account indefinitely and apply to whatever plan you have (newer Mobile Share/Mobile Share Value plans do __% off the data portion, older Nation/FamilyTalk plans do __% off the minutes portion).
The only change if you did a phone on Next would be the cost divided by the length of your term to keep the phone (Next 18 has your phone paid off in 24 months) added to your bill until it's paid off ($400/24) and then your rate goes back to your month-to-month amount. You can also pay off a Next phone early (really at the end of the day, it's a 0% interest loan). If you buy your phone outright, your service will also remain the same, but you won't see your phone on the bill (since you already paid it).
If you have a really really old plan, going to the Mobile Share Value plan may save a bit because it does discount for phones that are paid off, brought by you, or that you are paying for on Next. A friend of mine had an old plan that didn't have this provision, so she would've been paying the same rates _plus_ her Next phone charge...yuck. Converting to a current plan saved her and the other person on their line $20/month, so it certainly can be beneficial.
In terms of the SIM, all phones since the 5 use the same sized SIM, you can just swap it between devices. However, AT&T has been issuing new SIM cards since (I think) the 6 to add voice over LTE (VoLTE) provisioning and I think those cards are also designed for phones that need special SIMs for NFC payments. The iPhones don't, but basically AT&T treats everything newer than a 5s differently. It shouldn't be a problem to get the SIM swapped in the store - they sometimes do that as a troubleshooting measure and shouldn't charge you.
Sorry for the long reply, but I figured that might at least point you in the right direction or give you some things to ask/mention if you end up with a less-than-helpful individual at the store.
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Going to order new iPhone SE 64GB model. My iPhone 5s is a Verizon phone that came unlocked to run on any GSM like at&t. I really like that I can use CDMA or GSM. So if I buy the Verizon iPhone 5 SE will it also come unlocked for GSM carriers? Or should I order the sim free version, or does that only come with GSM? Thanks for help before I order.
If I remember correctly, Verizon had to make their phones unlocked for GSM use because of some FCC-related concession (buying spectrum or something else), so the SE should be no different. Basically at this point, the SIM-free one is the same model as the AT&T/T-Mobile/Verizon version (although locked on AT&T/T-Mobile) and the Sprint one is separate. They all have LTE/CDMA/GSM capabilities, but the Sprint one operates on different LTE frequencies:
http://www.apple.com/iphone/LTE/#iphone-se
