In regards to what fare it books into is a good question, I do know I didn't get the usual 50% bump in MQM's for an M fare or higher and didn't want to push the issue with Delta since I was so happy to be able to upgrade for 300Euros.

The key is to ask the KLM agents / staff, if you ask anyone associated or working with/for Delta they will deny this upsell program exists.

When you go from EWR to AMS, go in the KLM lounge in Schiphol or to the Transfer Desk and they will look at your fare basis and see if there is inventory and tell you the fee to upgrade, also make sure to have your electronic one way system wide certificate number ready. You can do this upsell I believe starting 6 hours before the flight departs, last time I did it 1 hour before departure.

In regards to which flights / metal it works on, KLM doesn't care if you are on their stock or metal or delta stock and delta metal, they can do the upsell. I have done this upsell on delta stock and klm plane, delta stock and delta plane, klm stock and delta plane as well as klm stock / klm plane. so yu can buy the least expensive ticket for the flight you want and still do the upsell.

What I haven't done yet is find a KLM agent in JFK that can do the upsell. I find it curious they can do it in AMS all day long but here in the US it's not even acknowledged.

Let me know if anyone else has success with this.