Originally Posted by B7e7US

You need to book it on a DL ticket stock (006) with an Air France flight number on S or W class. Calling DL and doing it on the phone is the easiest way, because if you purchase a DL flight on W, the agent will have to change it to an AF flight number and some agents are reluctant to do this as it seems to be a little bit complicated. Also, there's a luxury tax that needs to be paid before the upgraded ticket is reissued.

This thread can help you a lot:
http://www.flyertalk.com/forum/delta...rance-klm.html