Terry, You raise an excellent example of why setback lines (zoninig regultions) should NEVER be shown on a subdivision plat!!! Zoning regulations are just that... regulations. Regulations are prone to change over time and are subject to variance grants, etc. Setbacks are not boundaries. They are lines of regulatory interests which impact property.
As such, when the zoning setbacks were changed, their change impacted all property within the zoning district. Particular parcels (lots in your subdivision) cannot be singled out for different regulations than their neigbors within the same district. With that in mind, I'd agree with the Zoning Office and their Solicitor (regulation enforcement officials) that the current regulations would be imposed on new construction just as the current regulations would have been imposed on the date the construction was made.
Of course, an individual property owner can choose to increase their own setback, but they have no regulatory interest in making their neighbor do so. There would be an exception, of course, for any covenants, conditions or restrictions filed against the property. The question there arises as to who has the enforcement rights of CC&R's. Likely, it's the same agency officials who will use blanket agency rules.
JBStahl, PLS, CFedS
***May your Boundaries Fall in Pleasant Places*** Ps. 16:6