The question was around enhancing/replacing the existing maps, not just changing the map graphics.
Here are a series of shots which probably illustrate my point;
A quick look at the Marketgarden map shows that it is nothing like what we have in the expanded France 40 map.
Essentially, the original map has Eindhoven, Nijmegan and Arnhem lined up from south to north. I'm guessing that the original map was rotated so that up is not north, but rather north north east.
Here is the source map that the new France '40 map used. Note where Eindhoven is (all images can be right clicked for full size);
Here are the game maps side by side. The left hand map is the original Marketgarden map, the right hand taken from the France '40 map. You can see this first map that the locations are roughly equivalent on both (all images can be right clicked for full size);
On the next two maps, you can see how the maps diverge. Look at the different coloured circles. They denote the same town. Essentially, Eindhoven is off the new map to the left.
This final map has Eindhoven centered so that the relationship of the towns can be shown;
So as you can see we can't just substitute one map for the other, but would have to lay out every scenario again. That's not an inconsequential exercise. That said, the new map is much better (see the full river at Arnhem?) so we will see...