MSPeconomist, that's not it. This is not the same as changing the ticket (paying the fee + fare difference, and then miles to UG). The fare difference is not €400. FB members have this benefit (not sure if it is limited to Elites): ONLY at the airport the day of your flight, you may ask if there is space available for an UG. If there is, depending on your fare class you pay a SET AMOUNT to make your ticket upgradeable + the miles it takes to UG. The amount you have to pay depends on the fare that you bought (the cheaper the fare you bought, the higher the fee - if you search the FB forum should be able to find exact amounts). The FB forum should have more info on this. AFAIK, this was only available at AMS (if you flew AAA-AMS-BBB you could only UG the portion that originated in AMS as it the process was not available at outstations).

Because the way the program is set up, it doesn't encourage people to buy cheap fares in order to UG with cash+miles at the airport instead of buying biz fares. Bear in mind that €400 + 25k miles was only one way, it's only if there is space available, the flight isn't oversold in Y and (it was) only for flights originating in AMS (now possibly extended for other O/D flights).