Tile replacing

[Googi]

So say I want to replace all bushes and swamp in a level with normal grass tiles. Both bushes and swamp patches are comprised of 4 different tiles (8 in total) so using fill is not possible. I want to do this for several levels so using the level editor to replace them would be too time consuming. So I write a loop to check if each tile in a level is one of the 8 different tiles I want to replace, and if so it uses "tiles[x,y] =" to replace it. This works. The problem is that when I hit F4 and go back to the level editor, all the replaced tiles have reverted to their original form. Is there a way around this? If there isn't, there should be.