Assessable mutual insurers shall be subject to a cap on net annual premiums on the same basis and in the same manner as provided in former s. 624.469 as to commercial self-insurance funds. For an assessable mutual that has converted from a commercial self-insurance fund, the first 6 full calendar years of its operation as set forth in former s. 624.469 shall be computed from the date of its certificate of authority as a commercial self-insurance fund.