Conditional uses are those uses which may be appropriate, desirable, convenient or necessary in the district in which they are allowed, but which by reason of their height or bulk or the creation of traffic hazards or parking problems or other adverse conditions may be injurious to the public safety, welfare, comfort and convenience unless appropriate conditions are imposed. Uses designated in this chapter as conditional uses may be permitted, enlarged or otherwise altered upon authorization by the Planning Commission in accordance with the standards and procedures set forth in this subchapter and 157.229. In the case of a use existing prior to the effective date of this chapter and which is classified in this chapter as a conditional use, any change in use or in lot area or any alteration of the structure shall conform with the requirements dealing with conditional uses.