The exported .fhm file includes everything that is what sort of texture the map uses, which objects that go in certain places and in general, anything how it "should" look like. While the object files ".fho" is more like the source of what they are in person, what your game will look for when displaying the map for you.
There's a few different ways to make a more "winter" like map if I will be honest and it depends on how the map creator wants the edits to take effect and what the other users would need to download.
One way would be to already have object groups with both winter and summer meshes in them with different textures, and then when the time comes to it, just go into the folder for said map they're working on, and open the "mapnameobjects.cfg" file in notepad or other word editing program. In there you can pretty much just change the name of said name of a saved object in an object group, and that way do minor effort to change a look of a certain mesh placed all around in a map. GRANTED, that the object you replace the old one with has the same size and placement in the object maker and were a part of earlier .fho files given. Users would only need the new .fhm file of the map in that case.
The other way I guess would be to actually replace the objects in the object groups with the same mesh, but with a different texture applied to it. Once that has been done, and the map has been re-exported again, you'd have new .fho files with new winter textures, but it means they'd also more than likely need to send the .fhm file as well if it had a ground texture change and what not.
Hopefully this should make sense if you've been mapping a bit.
