Before you book you can call Delta and ask if there is upgrade availability for your flight(s). If there is and you have an upgrade certificate then it may be worth to buy a M fare.
If there are seats available in D1 on the DOD @AMS KLM will sell upgrades a fixed amount of EUR (or miles/GUC's or miles+co pay..but it might be n.a. for you after looking at your programs note).
Count on something around 800EUR.
Edit:
ex-AMS to :
€749 ATL
Last edited by Grouchy; Jun 1, 2015 at 1:35 pm