Wash. Rev. Code § 51.16.090
To the end that no employer shall evade the burdens imposed by an unfavorable or high cost experience, the director may determine whether or not an increase, decrease, or change (1) of operating property; (2) of interest in operating property; (3) of employer; (4) of personnel or interest in employer is sufficient to show a bona fide change which would make inoperative any high cost experience: PROVIDED, That where an employer is now or has prior to January 1, 1958, been covered under the provisions of this title for a period of at least two years and subsequent thereto the legal structure of the employer changes by way of incorporation, disincorporation, merger, consolidation, transfer of stock ownership, or by any other means, such person or entity as legally reconstituted shall be entitled to a continuation of the experience rating which existed prior to such change in the employer's legal structure unless there has been such a substantial change as provided in subdivisions (1), (2), (3) or (4) of this section as would warrant making inoperative any high cost experience.
RCW 51.16.090