Promotion can't be applied retroactively???!!!

Had two old Pixel 6a's on my account. Went in to the store on a Monday night to upgrade. My one daughter wanted an iPhone 17 the other a Pixel 10 Pro. We processed the iPhone. Then we processed the Pixel in the store. We were told that the 6a was a $27 credit, fine whatever so we did that. After that my daughter decided she did not what the iPhone 17, but it was too late to cancel so fine, I get the phone from FedEx and we then go to the store on Saturday to upgrade her to a Pixel 10 Pro. (Since her number was now locked to the iPhone 17.) During the process of this we are told that she can have the $27 credit or... she can have $400 if we go upgrade the line to Unlimited Premium, which if we do will be $8 a month less. So we do that. Of course my next question to the store was, why weren't we able to do that to the phone we did on Monday?? I was told to call support and ask. 

Which I just did, and was told that the only way to do this would be to send the phone back??!! WTAF? That makes ZERO sense. Something about their system can process it without sending the phone back. That is the most ridiculous thing I have ever heard. Why do I need to swap phones? You should be able to just make that change in your system. I have been an Xfinity Mobile customer since the first year it came out, guess I'll be rethinking that. How can you have a billing system that doesn't allow you to make a change like that without having to send a new phone and send one back? That is asinine.