Bought a new phone using an upgrade on different phone on my account than the number it would be used for. Mayhem ensued.

You can't. You chose to upgrade on that particular line so that's where the installment plan should be. This isn't a problem caused by AT&T. 

If you are still within the 14 day return period, you should absolutely return the phone and purchase it on the line you want the installment plan on. The small restock fee is much better than paying for a line you don't need for 36 months. If it's been longer than 14 days, then the installment plan stays where it is. You could disconnect the line still in use and change your phone number to your mother's number, or port your number to Google voice and have it forwarded to your mother's number. 

On another issue, the original installment plan shouldn't have affected using the new phone on your number. All you would have to do it switch the SIM cards. 

No physical SIM for iPhone14 Max Pro. And, I've done this switch approximately 3 times before (possibly more) with upgrades each year for my phone and my husbands and handed down ours to our two kids. So, we did the switching of the plan/line once every two years. And, technically, I would argue it was AT&T's fault if they allowed multiple times in the past and *never* said it wasn't allowed.

The part I don't understand is you upgraded an unused phone line because it had an upgrade available, now you're not happy that the installments are permanently tied to that line, which is exactly what should happen.  That's how all this works. 

If you wanted to cancel your mother's line, you should not have used it to upgrade your phone. You should have paid off your line and upgraded your line which is what you were forced to do anyway. 

If you aren't expecting/getting promotional credits for trade in, go ahead and cancel your mother's line. Your next bill will include the balance of the phone installments on the iPhone that you just purchased.

So, it was the process we used for years. What's to not understand? My mother's inability is a new development. It's how dementia happens.

I'm very sorry about your mother's condition.  My mother is 85 and has suddenly begun forgetting things, so I understand what you're going through.

Since you have already contacted at AT&T about moving the installment, you've already been told no.

In the past borrowing upgrades was something you did, and could do. I did it because my ex-husband wasn't a fan of change and would keep his phone forever.  But you couldn't cancel the line afterwards which is what you're asking to do.  The installments or contract would stay on the line that was actually upgraded, but the phone would be used on another line. The only wrinkle here is you now no longer need the line that was upgraded.

As I wrote the easiest thing to do is to go ahead and cancel the line and expect to have to pay off the remainder of the phone on your next bill.  

@formerlyknownas I appreciate the more thoughtful response (I was beginning to think that I was going to get a barrage of internet know-it-all created messages). I understand your original comment in the way it was intended now. Thank you. 

I do think that I might just have to pay it off in full, and probably will. I just wish AT&T could convey changes a bit better or more clearly. This falls under the "easier said than done" category, though, I'm sure due to number of unique situations/scenarios. 

Thanks again :-)

And, technically, I would argue it was AT&T's fault if they allowed multiple times in the past and *never* said it wasn't allowed.

What you did in here and in the past is certainly allowed. In the past, though, you've never wanted to cancel the upgraded line. That's the difference here.

I'm sorry if my earlier post wasn't clear. I usually communicate well enough, but not everyone communicates in the same way. I'm glad my second post explained it better in a way that made sense.