I have analyzed the Eden World Builder file format (see User_blog:Mrob27/Eden_World_Builder_v._1.7_Data_Format) well enough that I know how to modify Eden world files. Read that article for more information about my mapping program.
Today I tested some simple terrain modifications.
Here I have transformed a 64x64 patch of water, grass, trees and rock to look like it snowed. The algorithm for this transformation scans vertically starting at the top, looking for the topmost block to see what it is. Clouds and Leaves don't count, but leaves are made white if they're on the top. When it finds Grass, Weeds, or Stone it turns half of it into Ice and the other half into white Dirt or Sand or Cloud. The top of Water is made into ice except if the water is "shallow". Most everything else is made white, but only the topmost block. (In the picture you can see the Flower blocks were simply turned white, this was a bug.)
Here's a higher view:
You can also see some transmogrified clouds. I told it to look for white cloud above height 60 and turn it into white lava.
Two more pictures: