I think there are a lot of variables when it comes to the upgrade offers. On a combined AF/KLM itinerary, MAN-CDG-JNB-AMS-MAN, I was offered an upgrade to the first leg on the way out. Going back, no offer - the cabin crew did announce the long haul part was absolutely full and I observed one free seat in economy.

Over the years Ive taken a few upgrade offers from AF themselves - always good value, but not going to happen if the flight is full. I'm guessing KLM are essentially very similar.