No you can use the same form for this quite easily, modifying the presentation to suit.
However, that adds a layer of complexity and if you are not sure about creating general purpose scripts and forms then do it in 2. Here is what the update would look like.
edit.php
// edit.php get and displays record for update
if (isset($_POST['id'])) {
$rid=$_POST['id'];
// query for the data
$sql = "SELECT * FROM table WHERE rid=$rid";
$result = mysql_query($sql) or die (mysql_error());
if (!$result or mysql_num_rows($result) != 1) {
echo 'database error, record not found';
break;
}
$row=mysql_fetch_array($result);
echo '<form name="update" method="post" action="update.php">
<input type="hidden" name="rid" value="' . $row['rid'] . '" />
<input type="text" name="forcast" value="' . $row['forcast'] . '" readonly /><br />
<input type="text" name="actual" /><br />
<input type="text" name="circumstance" />
<input type="submit" name="submit" /></form>';
then update.php
if (isset($_POST['submit'])) {
$act = $_POST['actual'];
$circ = $_POST['circumstance'];
$rid = $_POST['rid'];
$sql = "UPDATE table SET actual=$act, circumstance='$circ' WHERE rid=$rid";
$result = mysql_query($sql);
}