Under MGL C. 40, § 6, lots not held in common ownership with any adjoining land are generally not subject to subsequent amendments in dimensional requirements, and land shown on subdivisions or other plans endorsed by the Planning Board are exempted from subsequent zoning amendments in certain respects for a limited period of time (See MGL C. 40A, § 6). Building upon, but not subdivision of, any lot, if nonconforming but having an area of 5,000 square feet or more and frontage of 50 feet or more and held in ownership separate from all adjacent land, shall be governed by those lot area, frontage and side or rear setback requirements applicable to it at the time of its recording by plan or deed as a lot in separate ownership, except in cases where those requirements have subsequently been reduced through amendment of the Zoning Bylaw and in cases of building for multifamily dwellings, in both of which cases current requirements shall apply. Nonconforming lots may be changed in size or shape or their land recombined into fewer lots. In doing so, such lots will not lose this exemption from current requirements, so long as the change does not increase the frontage or area of a conforming lot by reducing the frontage or area of a nonconforming one.