(in 1977, plaintiff entered into Williamson Act (law establishing conservation contracts with landowners limiting land use to use for agricultural purposes in return for lower property tax valuation) contract with landowner establishing minimum parcel size of 160 acres for subsequent divisions of the property which was consistent with agriculture preservation Guidelines then in effect; in the following year, plaintiff revised guidelines and increased minimum parcel divisions to 600 acres; defendant purchased subject property and then divided and sold much of the land with some divisions being less than 600 acres (but all were more than 160 acres; plaintiff’s Guidelines upheld on appeal – plaintiff and prior owner voluntarily renewed their contract numerous times; new contracts incorporated revised Guidelines and revised Guidelines can be applied constitutionally to a Williamson Act contract executed in 1977).