Actually, this bug occurs if any partial improvement is attempted and not finished prior to Terrain Type Conversion.
I visited a Terrain Tile after several turns that was de-forested (was Forest, now Plains) and where i had partially completed a Road improvement during it being a Forest Tile, and i still had to use 6MPs to build a Road, it held the fact that i was building a Road on a Forest Tile. I would expect the Road MPs to be converted to the new Plains Tile (3MPs, not 6MPs) or cleared altogether when the Tile de-forested.
Posted 12-Jan-10 01:15 by Cor'e =)