I have looked around and performed some searches in this forum but have not found this mentioned elsewhere. (C+ is hard to search ) If there is already a thread, mods, please merge.
In looking for some C+ purchases for travel next year, I started to see 'S' class offered as a Comfort+ option here and there.
In looking around, I found this:
Delta Comfort Plus | Delta Professional
Delta Comfort+ fares are booked in "W or S*" Class. *Delta Comfort+ fares in “S” class will be filed in markets that offer Delta Comfort+ as a fare through the month of July, for travel on/after September 4, 2019.
This has several ramifications - not so much with Skymiles as MQM earn is still 100% as is with W.
This change may make SDC more complicated if it becomes class based (e.g. Booked in S but only W available).
It could change things with some partners, unless the earn tables are updated. For example with Virgin Australia Velocity, S on Delta currently earns as "Discount Economy" while W earns as "Premium economy".
It seems upgrades may be rebooked into S class as well.
Upgrade Notification - “S” Class will be a valid class used for Delta Comfort+ as a fare and when a customer is upgraded into Delta Comfort+.
- Medallion members will be notified of their new Delta Comfort+ seat assignment via email and will have the option to change to another Delta Comfort+ seat, if available.
- For travel agency booked PNRs: When an upgrade occurs, “S” Class will be a valid class used when a customer is upgraded into Delta Comfort+ (“W or S” class is still used for paid purchases, either as a fare or an ancillary seat product).