305.7. If an effect changes a land’s subtype to one or more of the basic land types, the land no longer has its old land type. It loses all abilities generated from its rules text and its old land types, and it gains the appropriate mana ability for each new basic land type. Note that this doesn’t remove any abilities that were granted to the land by other effects. Changing a land’s subtype doesn’t add or remove any card types (such as creature) or supertypes (such as basic, legendary, and snow) the land may have. If a land gains one or more land types in addition to its own, it keeps its land types and rules text, and it gains the new land types and mana abilities.

10/4/2004: If an Assembly-Worker is changed to another land type, it will stay a 2/2 artifact creature and keep the creature type "Assembly-Worker" (until end of turn), but it will change its land type as directed.