Reconstruction or alteration. A nonconforming building may not be reconstructed or altered during its life to exceed 50% of its fair value, unless such building is changed from a nonconforming use to a conforming use as defined by this chapter, except that a mobile manufactured home which is a preexisting nonconforming use may be replaced with a new or larger mobile manufactured home, provided that such exchange is made within 30 days, and the owner has obtained a building permit to make the exchange.