Historical Rules · 1 October 2010
Rule 204.1a
Official source text from the 1 October 2010 Rules release
When an object's card type changes, the new card type(s) replaces any existing card types. Counters, effects, and damage marked on the object remain with it, even if they are meaningless to the new card type. Similarly, when one or more of an object's subtypes changes, the new subtype(s) replaces any existing subtypes from the appropriate set (creature types, land types, artifact types, enchantment types, planeswalker types, or spell types). If an object's card type is removed, the subtypes correlated with that card type will remain if they are also the subtypes of a card type the object currently has; otherwise, they are also removed for the entire time the object's card type is removed. Removing an object's subtype doesn't affect its card types at all.