[DRG]

In game data / design there is a terrain Terrain Tile Index

The game "knows" it is looking for Green grass in TER00. If you swap TER00 for TER10 - Snow you will see snow where there was flat green grass but for one that will only change the flats....hills are another shp file and that is only clear grass terrain but the game looks for the SHP file it does not save that SHP file data in the map save....just that it needs to use that particular shp file so you could go through the lengthy process of swapping the Terrain SHP's around but if anyone else tried to that map in their game it would be green because their grass is still the correct TER00.

You could experiment with a simple flat map.

FIRST MAKE A DUPLICATE INSTALL OF THE GAME

.build that map with standard ground level 0 clear green grass (generate a map and then press the CLR button and then save )then rename TER00 to XTER00 and rename TER10 - (Snow) to TER00 then run that map in the game...should see that green grass as snow. but you will also see about 20% as black voids ( see below).....save that version of the map then put everything back to normal... change your altered Ter00 back to Ter10 and remove the X from your Ter00 and load that map and it will be back to green grass

However, none of that will work exactly as green grass has more basic tiles in its SHP file than snow does so what you propose is impossible even when just displayed in your own game as there will be a lot of black voids