Unless things have changed, note that an upgraded card still retains its original anniversary date, and certs are tied to that date, not the upgrade.
All of this is in the T&Cs.
https://www.americanexpress.com/us/c...card/36182-9-0
scroll to or search for "Delta Reserve Companion Certificate at Renewal" and in the middle of that paragraph: "Companion Certificate valid for one round-trip First Class, Delta Comfort+®, or Main Cabin Companion ticket with the payment of applicable taxes and fees detailed below and the purchase of certain adult round-trip fares on published routings within the 48 contiguous United States. Residents of Hawaii, Alaska, Puerto Rico or the United States Virgin Islands (USVI) must originate from there to the 48 contiguous United States and have an address on their SkyMiles account in Hawaii, Alaska, Puerto Rico or the USVI. Each year, the Companion Certificate will be made available for redemption on delta.com in your renewal month. ... Tickets are only available in I and Z classes of service for First Class travel, and only available in L, U, T, X, and V classes of service for Main Cabin travel. For Delta Comfort+ travel, tickets are available in W and S classes of service, but only when L, U, T, X, or V classes of service are available in the Main Cabin."
Read back in this thread for usage notes; I think it's still the case that, despite the terms, the website does not let you book into Z and a lot of agents have a misconception about it too, so be prepared to argue for it
