The Governing Body of the City may, from time to time, in the manner hereinafter set forth, amend the regulations imposed in the Districts created by this chapter, amend District boundary lines, provided that in all amendatory orders adopted under the authority of this section, due allowance shall be made for existing conditions, the conservation of property values, the direction of building development to the best advantage of the entire City, and the uses to which property is devoted at the time of such amendatory order. In case, however, of a protest against such change duly signed and acknowledged by the owners of 30% or more, either of the areas of the land (exclusive of streets and alleys) included in such proposed change or within an area determined by lines drawn parallel to and 185 feet distant from the boundaries of the District proposed to be changed, such amendment shall not become effective except by the favorable vote of 2/3 of all the members of the Board of Aldermen.