passing down devices among users on the same account

No matter how you do the purchase you're going to pay $829 plus tax. You'll either pay an upgrade fee through AT&T or you'll pay $30 more buying directly from Apple at full price. If you purchase from Apple you are purchasing it unlocked device.  AT&T devices are locked and will remain locked until they are paid off and you request the phone be unlocked through the computer portal.

The wrinkle here is the iPhone 14 has eSIM only.

It's not really that big a deal but if you are used to moving a physical SIM card from phone to phone, this is different..

When you get the iPhone 14 you will want to transfer the phone number from your son's phone first.

https://support.apple.com/en-us/HT210216

You can then do the same thing to transfer your husband's phone number to the iPhone 13.

It really doesn't matter how you purchase the device if you're paying for it outright. Keep in mind that most AT&T stores won't sell them that way, typically requiring an installment plan to pay devices off over time. Once you have the new iPhone 14 new SIM cards may not be required and you can just move Son's SIM into 14 and Spouse's SIM into 13. If new SIM cards ARE required, then you'd take them to an AT&T store and pay for new ones. $5 each typically.

Hmm. Interesting.

Apple sells directly for the AT&T customer with a pay full price option.

I have no trade in so there is no benefit to using installment.

By selecting AT&T as my provider and the phone number on my account that I'm upgrading, I get the $799 price. If I do not specify a carrier and phone number that out the door price jumps to $829. (prices are for a basic 14 with 128gb)

Moreover - I thought that the 6s sim would not fit the 13 sim slot... AND it is not a 5g sim... Yes or No?

Oops. It seems the 6s uses the same size sim as the 13. Mea Culpa!

So I suppose the way to go is upgrade the 13 to the 14 so the SIM to eSIM migration is done cleanly. Then I just slip the SIM from 6s into the 13 and we are good to go.

Ah yes, I was forgetting that the iPhone 14 is eSIM only. I think you've got it though. If you have issues with connection after moving from 6S to 13 then a new SIM is in order.

Then I just slip the SIM from 6s into the 13 and we are good to go.

Thats fine for moving the number but you’ll also want to move the wife’s data from the 6s to the 13. Best way to do that is first erase all content and settings on the 13, then set it up using the 6s as the data source.

Thanks sandblaster. Normal transfer process as we've had in the past.

After that data migration completes, I am hoping that the nice folks at AT&T can re-provision that 13's 5g SIM to her old phone number. Then we'll put it back into the 13 for 5g service. fingers crossed.

@fosh if you transfer the number from 6s to esim on the 13, you don't need to provision anything. 

Ah ha! This is good to know.

Thanks @formerlyknownas