Unfortunately as someone who isn’t affected I can’t speak with 100% certainty but it seems to be a case where if the country your account was registered under does not match the billing address of your payment method, the payment won’t be accepted. So if you change country at all, you’re at the mercy of what alternative options are available there that do not require billing addresses, which in some places is virtually none.

You shouldn’t be forced to jump through hoops in the first place especially if you’re still in the same region the account and game key is under. But as it stands you literally cannot update your billing address country so you’re SOL if you’ve moved at any point in the last however long you’ve had a SE account.