I have found, especially with C+ upgrades at booking, often the UG is available for you to select the new seat before you get the email. Meaning, I buy a Y ticket. Complete the purchase and then go back in and select C+ seats. I'll get the email an hour or so later. I've found F upgrades work similar, where if I check at my window I can sometimes select the F seat and then the email comes. However, the big key is open inventory.