It looks like there is a lot of confusion about the administrative fee for a fare decrease for international travel. Its been a good 6 months or more since my fare decrease for international travel, and at that time, the fee was $50. However, after doing a search, there is more fuel to the fire to suggest the fee is now $100:
http://flyertalk.com/forum/showthread.php?t=653648
http://flyertalk.com/forum/showthread.php?t=722222
http://flyertalk.com/forum/showthread.php?t=733527

So, I guess you'll have to call NW and find out. I'd try a few times to see if they all say $100 for international. I'm sure domestic is $50, so maybe an agent will give you that fee.