Any nonconforming use of a structure may be changed to another nonconforming use, provided the new use is not a substantially different use, as provided below and approval for such change is granted a special permit by the Board of Appeals. For the purpose of this section, a substantially different use is a use which by reason of its normal operation, would cause readily observable differences in patronage, service, sight, noise, employment or similar characteristics, from the existing nonconforming use.