If a change of organization or reorganization would result in the annexation to a city of land that is subject to a contract executed pursuant to the Williamson Act (Chapter 7 (commencing with Section 51200) of Division 1), the commission, based on substantial evidence in the record, shall determine one of the following:
(a) That the city shall succeed to the rights, duties, and powers of the county pursuant to Section 51243, or
(b) That the city may exercise its option to not succeed to the rights, duties, and powers of the county pursuant to Section 51243.5.