well that's easy...
$sql = "SELECT Current_Value, id FROM stock_index";
$result = mysql_query($sql) or die(mysql_error());
while ($row = mysql_fetch_array($result)){
$current_value = $row[0];
$id = $row[1];
$sql2 = "UPDATE stock_index SET stock_index SET Starting_Value= "$current_value" WHERE id = $id"
$res = mysql_query($sql2) or die(mysql_error());
}