Where any existing antenna supporting structure is made nonconforming by implementation of this Section and its owners or operators seek to make a modification that substantially changes the physical dimensions of the tower or base station, any such modification shall require both the existing supporting structure and the modification to be in compliance with § 10.17: WIRELESS COMMUNICATION FACILITIES of this Code. The owner or operator of the facility may apply for a variance and further demonstrate that the requirements of this Section impose an unreasonable burden on the ability of a provider to provide personal wireless services pursuant to a license from the FCC or by reason of federal law.