I also have tried to book a higher fare, i.e. up to a K domestic to use PMUs and been told there would be a $150.00 change fee; this is for the "exact same itenerary". This is a very touchyt area for me personally, and I have called and written more than once. Maybe we need to make a full court press on this one by our fellow FTers and see what happens!
Would be nice if:
a: DL would waive this to get more revenue on same itinerary, and/or
b: consider at least reducing it for their most $$ frequent flyers (Platinums/Golds)