When the regulations for a zoning district established by this title contain performance standards, any permitted or special use constructed, established, altered or enlarged after the effective date hereof shall be operated so as to comply with the performance standards for the district in which it is, or will be, located. No permitted or special use already established on the effective date hereof shall be altered or modified so as to conflict with, or further conflict with, the applicable performance standards established for the district in which it is located. (1979 Code § 5-3-3)