No building or structure shall be erected, substantially altered, changed in use, or any land used or changed in use unless adequately maintained off-street parking spaces, either in garages or open parking areas, and off-street loading spaces have been provided in accordance with the provisions of this chapter. The provisions of this chapter shall not apply to any building, structure, or land use existing before the effective date of the Regulation or any amendment thereto unless such building, structure, or use is altered or changed. However, the number of off-street parking or loading spaces shall not be reduced to an amount less than required for a new land use as specified in this chapter.