Wash. Rev. Code § 80.16.050
The commission shall have continuing supervisory control over the terms and conditions of such contracts and arrangements as are herein described so far as necessary to protect and promote the public interest. The commission shall have the same jurisdiction over the modifications or amendment of contracts or arrangements as are herein described as it has over such original contracts or arrangements. The fact that a contract or arrangement has been filed with, or the commission has approved entry into such contracts or arrangements as described herein shall not preclude disallowance or disapproval of payments made pursuant thereto, if upon actual experience under such contract or arrangement, it appears that the payments provided for or made were or are unreasonable. Every order of the commission approving any such contract or arrangement shall be expressly conditioned upon the reserved power of the commission to revise and amend the terms and conditions thereof, if, when, and as necessary to protect and promote the public interest.
RCW 80.16.050