Based on most recent experience (and this isn't the first!), you should avoid booking flights with Delta where you end up on KLM. These folks really haven't got their act together. It's almost impossible to reserve seats, regardless of your status. When you call Delta, they tell you that you need to contact KLM; but when you contact KLM they tell you that they cannot do anything because the ticket was issued by Delta! Forget about upgrades - you cannot use system-wide vouchers or miles and the lowest status Flying Blue outranks Delta Platinum Medallion at the gate.