This above is exactly what i did to be able to use my existing PoGo account with a new Apple ID, except, I used an old iPhone to set up my new Apple ID.
1) I reset my old iPhone so it was completely wiped clean, because i didn’t want to log out of my Apple ID on my newer iPhone - I didn’t want to have to move the newer phone’s SIM to my old phone to get texts and email etc because then I’d have to set all that stuff up again on the old phone.
2) I followed the instructions to set up a new Apple ID on the old phone using a different, new, gmail email account, not the one I used for my PoGo account.
3) I used the same credit card as the original Apple ID account.
4) I then installed PoGo on the old phone with the new Apple ID, and logged into my existing PoGo account with my existing gmail account (that was not able to make any in-app purchases in the shop using the original Apple ID with the duplicate purchases on my newer iPhone)
5) I was again able to make purchases on my Pogo account.
so yes, the bug is on Niantic’s part, in that there is a purchase on that old Apple ID account that is trying to be sent to your pogo account, but it can’t be because of some programming error on Niantic’s part- in my case because it was because a glitch in Niantic’s code allowed a duplicate purchase for an event ticket that was only supposed to be allowed one time per each Apple ID on each Pogo account. This error locks up your Pogo shop.
and yes, Apple denied my request for a refund for a duplicate Gastly day bonus event purchase, and forced me to do this. It is clear there is a duplicate purchase on my device, but Apple doesn’t care. It is a ONE DOLLAR purchase, and I told apple to keep the money and just remove both purchases from my account, but they won’t do it. Even though I was charged for something I didn’t get.
I tethered my old iPhone to my newer one using a the personal hotspot option on my newer iPhone., and took both phones out to GoFest.
all this just to play PoGo and use my GoFest ticket THAT I ALSO PAID FOR. I am so beyond Apple’s support people at this point, Niantic needs to step up and eat the charges for the duplicate purchases, or the ones that didn’t go through, but apparently there is no way for Niantic to see which purchases made it to which pogo account.