Ambulatory

Movable; revocable; subject to change; capable of alteration.

An ambulatory court was the former name of the Court of King's Bench in England. It would convene wherever the king who presided over it could be found, moving its location as the king moved.

An ambulatory disposition is a judgment, decree, or sentence that is subject to change, amendment, or revocation.

A will is considered ambulatory because as long as the person who made it lives, it can always be changed or revoked.