If it's 30 or more days BEFORE the election, a voter who has moved should update their registration, either online or on a paper form (they must first update their address with the SC Department of Motor Vehicles).
If it's less than 30 days until the election, a voter CAN vote IF they:
Moved to another residence within their precinct, they can update their address at their polling place and vote a regular ballot. (Precincts can be found at scvotes.com) NOTE: For voting purposes, their actual address does not have to be the same as the address on their driver's license or other photo ID. This is true UNLESS they have moved to another county or state.
Moved to a different precinct within their county. They will have to vote Failsafe (see below). It's not the best option, but if it's too late to update registration, Failsafe is better than not voting.
If it's less than 30 days until the election, a voter can NOT vote IF they:
Moved to another residence in another county prior to the 30-day registration deadline and did not update. They are not eligible to vote UNLESS they updated their address BEFORE the 30-day deadline.
Moved to South Carolina after the 30-day registration deadline. They are not eligible to vote. They can check with their previous state of residence to try to vote absentee in that previous state.
FAILSAFE VOTING
A failsafe provisional ballot contains only federal, statewide, countywide, and municipality-wide offices, not local ones, because a person who moved may not be eligible to vote in for offices in a different precinct.
To vote Failsafe, a voter can choose one of these options:
Vote at the polling place in their previous precinct using a failsafe provisional ballot.
Go to the voter registration office in the county in which you currently reside, change your address, and vote a regular ballot there.
(From https://scvotes.gov/voters/voter-faq/)