No structural alterations shall be made in a building or other structure substantially occupied by a non-conforming use, except when made:

1. in order to comply with requirements of law; or
2. in order to accommodate a conforming use; or
3. in order to conform to the applicable district regulations on performance standards; or
4. in the course of an enlargement permitted under the provisions of Sections 52-41 to 52-46, inclusive, relating to enlargements or extensions; or
5. in the course of an alteration to improve the energy performance of the building or other structure, including, but not limited to, the addition of solar energy systems, energy storage systems, or qualifying exterior wall thickness;
6. or except as set forth in Sections 52-81 to 52-83, inclusive, relating to regulations applying to non-conforming signs.