A quick note that occurred to me on the topic of the impossibility of changing terrain types. When Sizemore made that comment
about modifying the terrain type, he was referring to how the Volcano Uncroaking in one action turned the terrain of at least part of Gobwin Knob from mountain (or extinct volcano, if that's an applicable distinction) to active volcano. A change like that would be almost impossible in our world with our technology, akin to turning desert into forest. It's a change that the world as a system is unlikely or unable to produce naturally, and Sizemore (in the link) did it in a single turn.
That's not to say that changing terrain types isn't impossible, that things like deforestation and desertification don't occur in Erfworld, just that Sizemore's statement isn't necessarily expressing an absolute condition.