Any nonconforming use of a structure may be changed to another nonconforming use, provided that the changed use is not a substantially different use, except as provided in Subsection B below, and approval for the change is granted by a special use permit for an exception by the Board of Appeals. For purposes of this section, a "substantially different use" is a use which, by reason of its normal operation, would cause readily observable difference in patronage, service, sight, noise, employment or similar characteristics from the existing nonconforming use or from any permitted use in the district under question.