Resolutions passed under authority of this act shall not apply to the existing use of any building or land, and shall not prevent the restoration of a building damaged not more than 50% of its structural value by fire, explosion, act of God, or the public enemy, or prevent the continuance of the use of such building or part thereof as such use existed at the time of such damage but shall apply to any alteration of a building to provide for a change in such use or a change in the use of any building or land after the effective date of any such resolution.