1) I think I heard that if any changes are made (like payment changes), it’ll basically drop the GCU discount. Is this true?
Yes, this is true. This just happened with my Shovel Knight 3-pack of amiibos that I ordered October 2017. It was tied to a credit card that has since expired. I got an email on last Friday to update my payment method. I did so, and it processed the order at the full price of $34.99 instead of the $27.99 I ordered by when I had GCU. I had to call their Customer Service number to get a credit, but it was a real PITA. Not to mention I had to call anyways because their self-service for updating payment on the website wasn't even working. Lastly, the order got delayed from this Tuesday to this Friday because I also updated the shipping address since I moved recently.