As long as the old FSID was not deleted, but only merged on FamilySearch, it should update the FSID after visiting the person. Open them in the FamilySearch Person Tools then display someone else, come back to them and the new FSID should appear. If you need to UnMatch them you can click on that button on the Share Data tab then on the Find Matches tab find the person to link them to. If you have duplicates there you can check their box next to the possible match and review the match to merge them.