Originally Posted by
FrankTalk
This is more or less correct, the actual rule is KL/AF/DL metal DL code or DL metal DL/KL/AF code if memory serves (pro.delta is not loading right now for some reason, can't check). Sometimes they do make an exception and let you use AF/AF and such but even they need to get a waiver for that. However there's a workaround: have your ff number scrubbed from the res and retro claim in FB with the appropriate flight number.
That's from Bulletin 6, whereas I was just rebooking under the fare flexibility - not sure if that has the same rules. I think I actually booked the original flights for this all on KL codes - meaning presumably KL fares despite the 006 stock - but I didn't push on that front.