For anyone confused, here is exactly what I did to make it work. YMMV though I guess, apparently. Saw someone talking about adding things to carts, which is not what I did at all and I'm not sure how that would happen.
1. On my Xbox, went to the store app, searched "Diablo", selected the standard edition of Diablo IV, clicked "preorder", paid $69.99 plus tax on my normal credit card I have on file for xbox.
2. Waited about five minutes until I got the email from microsoft confirming my order.
3. On my xbox, closed the store app and re-opened.
4. On my xbox, scrolled down on the main store page (actually I can't remember exactly where it popped up but it was in the store app and not hard to find) found the "just for you" special offer to buy Diablo IV ultimate edition for $30. Clicked it, paid $30 plus tax, same way as above.
5. Waited another couple minutes, got another email from microsoft confirming.
6. On my laptop, inside the email confirmation (the one for ultimate but i dont think it matters) I clicked on "view or manage your order" and logged in to my microsoft account.
7. the link took me to a page containing my order history. Next to the entry for Diablo IV standard edition, I clicked "Cancel".
8. A minute or two later, I got another email from microsoft, confirming the cancellation, and a refund of the $69.99 plus tax.
9. Went back to my order history. It now shows an entry for Diablo IV standard edition as "refunded", and an entry for Diablo IV ultimate as "completed" and releasing on 6/1/23, with a total charge of $31.65.
10. Not a step, but just as an additional note, you can check on the order history page (on your phone or laptop) underneath "bundle includes" and it shows both the standard edition and the ultimate edition content.
Good luck everyone.