S.C. Code Ann. § 56-3-730

The owner of a vehicle classified under Sections 56-3-130 and 56-3-660 to 56-3-710, may register and license such vehicle for the desired load capacity classification, and a previous registration and license for a particular load capacity classification for a particular vehicle shall not be deemed as fixing the load capacity classification on such vehicle for future registrations and licenses or renewals. Also the owner may have the privilege at any time during a then current registration and license period to raise the load classification for a vehicle by making application therefor to the Department of Motor Vehicles and paying the required increased differential in fee, but the owner may not have the privilege of reducing the load classification for a particular vehicle until the expiration of the then current registration and license period.