Rulings

Terraformer’s effect overwrites the land type of all lands you control that had other types and gives a land type to all lands you control that don’t normally have one.
Changing a land’s type doesn’t change its name or whether it’s legendary or basic.
Lands you control will have the mana ability of the basic land type you choose (for example, Forests can tap to produce green mana) and will lose all other innate abilities they had.