I have found a way to transform forest into plain: select a lumber mill to be build on a forest, then destroy it before the construction has began. The forest is removed and you can't no longer build a lumber mill on this tile. But you can build any other improvement that can be build on a plain.
I am not sure that this is really working has intended.
I have stumbled on this behavior when trying to change the order in the city build queue by removing items.
