I think this has been discussed somewhere but it’s pretty clear you can use RUC still
- Flights with domestic Delta One and domestic Delta Premium Select:
- 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. If you apply your Regional Upgrade Certificate, then you will be upgraded to Delta Premium Select pending upgrade availability.
- If you purchase Delta Premium Select and then apply your Upgrade Certificate, you will be upgraded to Delta One, pending upgrade availability.