Couldn’t find this after searching here, and have been on hold with Delta for two hours so far:

I used a Delta Amex Main Cabin Companion Certificate to book a round trip flight with a friend. Once we booked, I paid the extra amount to upgrade to Comfort+. Now we have the opportunity to I’ve to another flight earlier in the day (eligible main cabin fare bucket opened up.). Changing flights would require us to switch to a main cabin seat since this was a Main Cabin Companion Certificate.

My question: if I make that change, what happens to the money I paid for the C+ upgrade? Do I get an e-credit that I can apply toward a similar upgrade on the new flight, or is it just lost? The FAQ on Delta’s page says post-purchase upgrades are “non-refundable” but I’m not looking for a refund per se, so I don’t know if that applies in this situation. It also says post-purchase upgrades must be used “as ticketed” or they are forfeit but, again, I don’t know what that means when I am only changing one flight on a ticketed round trip itinerary.

Any input appreciated.

Thanks,
Mike