Buildings existing at the time of adoption of this Zoning Ordinance which do not meet the off-street parking requirements, may be structurally altered to the extent of fifty percent of the cost of equivalent new construction, and the use of such buildings may be changed to an equally intensive or less intensive use without providing the required off street parking spaces. However, if such buildings are structurally altered to an extent greater than fifty percent of the cost of equivalent new construction, or if the use is changed to a more intensive use such as conversion of a single-family dwelling to an apartment or a change from a B-1 to a B-2 use, all required off-street parking spaces shall be provided.