Your previous code lacked the 'fileName' context given in your latest post. Accordingly, I assumed that was just your attempt to retrieve the existing file's name. Given that's not the case, you can use:
ActiveWorkbook.SaveAs "S:\Recruiting Information\Field Recruiters\Area Books\Mapping Files\" & Split(fileName, ".xls")(0) & " - Mapping File.xlsx", FileFormat:=xlOpenXMLWorkbook