Good question. From the Delta website:

"Complimentary Upgrades are not permitted to Delta Premium Select or Delta OneŽ flights outside of the U.S. 50."

Here is more information: https://www.delta.com/us/en/skymiles...llion-upgrades

Basically, the only way to upgrade to Premium Select or Delta One is to pay with cash (you may see an upgrade offer on your trip) or repurchase your ticket.

Unless you are become a Diamond Medallion and use Global Upgrades as one of your Choice Benefits:

https://www.delta.com/content/www/en...-benefits.html