There has to be more to this story. Sounds plausible enough when viewed from the point of view of the passenger paying for coach and upgrading to business. But try this hypothetical scenario:

A passenger has a paid, refundable business class ticket, ATL - AMS, Delta stock, Delta metal, fare is about $8,000. Can this passenger go to the ticket counter in AMS, purchase a one way coach ticket for the return, then upgrade it for 300 euro? Delta will be losing at least a couple of thousand $ in this case, I am having a hard time believing that they will let this situation stand.