Originally Posted by xliioper

Presumably you are talking about a domestic Delta One flight. Delta One flights definitionally do not have a First Class cabin. They may or may not have a Premium Select Cabin. If they don't a have a Premium Select cabin, you can upgrade directly from a Main cabin fare to D1. If it says "Delta One available with Upgrade Certificate", you can use either a RUC or GUC to upgrade. If it shows Global only with 2 pax, that means there's only one seat available in the upgrade bucket for RUCs (OY bucket). Upgrades on domestic D1 flights with GUCs only require Z inventory. They may or may not open additional OY inventory, but I wouldn't count on it. Yes, you can get the upgrade cert back if you decide to cancel.

Originally Posted by chipbennett

UCs work for an upgrade of one cabin class. Both Economy and Comfort+ are part of a single cabin (Main Cabin). So, booking in either one will allow for upgrade into the next available cabin class. Where the plane in question only has two cabin classes (Main Cabin and either FC or D1), the UC will work directly into FC/D1. Where the plane has 3 cabin classes (Main Cabin, PS, D1), the UC works from either Main Cabin (Economy or C+) to PS or PS to D1.

(Note: this is the reason that I'm pedantic about not calling Premium Select "Premium Economy": because Delta treats it as a cabin class separate from, and higher than, Economy.)