If you purchase Main Cabin or Delta Comfort+ and then apply your Global Upgrade Certificate, you will be upgraded to Delta Premium Select upon request and then will be added to the waitlist for Delta One and eligible to clear starting 24 hours prior to departure.

Yes, you can be upgraded to Delta Premium Select, but the rules vary significantly based on the route. For flights within the United States (such as transcontinental routes on specifically equipped aircraft), Medallion members and Delta SkyMiles Reserve cardholders are eligible for Complimentary Medallion Upgrades to Premium Select, following the same priority hierarchy as First Class. However, for international long-haul flights, Delta does not offer "complimentary" space-available upgrades to Premium Select for elite members. On these routes, you must use a Global Upgrade Certificate (GUC), which is a choice benefit for Diamond Medallion members, or you can use SkyMiles to "buy up" to the cabin after booking. Occasionally, Delta may offer "targeted" mileage or cash upgrade offers in the app. If a flight is overbooked in the back, operational upgrades (OPUPs) may occur, but these are rare and purely at the airline's discretion. In summary: domestic is potentially free; international requires a certificate or payment.