On Premises Sign ordinances Tenessee

I am trying to see if it is lawful for a municipality in Tennessee to require a business to remove a sign that has been in existence for years because the city changed the ordinances abut signage. I seem to recall that a sign is grandfathered under the ordinances that were in place when it was build until such a time as the sign is removed or abandoned. I am looking for TN state law that would define that a sign would be grandfathered and also hopefully define what would require a sign to be brought to current code.