No later than December 1, 1971, the commission, after public hearing, of which adequate descriptive notice is given, shall adopt and file with the Governor and the Legislature a resolution fixing and establishing within the shoreline band the boundaries of the water-oriented priority land uses, as referred to in Section 66602. After such filing the commission may change such boundaries in the manner provided by Section 66652 for San Francisco Bay Plan maps. Such change will become effective only if authorized by an affirmative vote of two-thirds of the commission’s members and, where the change involves a reduction or elimination of a priority use area which has been so designated because of contemplated acquisition necessary to implement the priority use, upon a finding that there is no substantial probability that a public agency will be committed to acquiring the area within a three-year period commencing January 1 of the year following the year in which such a finding is made. No other changes shall be made in such boundaries, except with the approval of the Legislature.