Terraforming? Help Kael!!

[Chaotic Law]

Ok, here we are again. I am a HUGE terraforming fan going way back. So, what I would like is info on removing the restriction of mountain being impassable and then maybe an example of how a worker can change a tile. My goal is to make it so a worker can change any tile to another tile type but through a long process. Like taking a desert to a plains then to a grassland. Or a mountain to a hill. Any info would be great. Thanks!

Chaotic Law

PS: I posted this in the 2K forums but got no solid answers.