<?php
$username="MY_MYSQL_USERNAME";
$password="MY_MYSQL_PASSWORD";
$hostname="MY_DATABASE_HOSTNAME"; //Your Database hostname usually
$database="MY_DATABASE_NAME";
$my_text = "test_post_meta";  // What I'm searching for
$my_category = '8';    // The category to change it to

// Connect to MySQL and the database and verify:
mysql_connect($hostname,$username,$password) or die(mysql_error());

echo "<p>Connected to MySQL.";
mysql_select_db($database) or die(mysql_error());
echo "<br />Connected to " . $database . "</p>";

// Verify what we're looking for, for troubleshooting:
echo "<p><b>Looking for " . $my_text . "</b></p>";

// Get the ID field (which is WordPress's post
// number) from any posts that have your text:
$query = "SELECT ID FROM wp_posts WHERE post_content LIKE '%$my_text%'";

// Take those results and go through them:
$result = mysql_query($query) or die(mysql_error());

// While there are results...
while($row = mysql_fetch_array($result))
{
// Verify what we're doing -- changing post
// number such-and-such...
$thisPostHasIt = $row['ID'];
echo "<p>Row " . $row['ID'] . " contains it, so...<br />";

// In the wp_term_relationships table,
// update the category number ("term_taxonomy_id")
// with the category number you specified -- but only
// in one of the "result" rows.
// We look for "object_id" to equal one of those
// rows. (The object_id field refers to the WordPress
// post number, just as the ID field did. Why two
// different names? Who knows?)

mysql_query("UPDATE wp_term_relationships SET term_taxonomy_id='$my_category' WHERE object_id = '$thisPostHasIt'");

// And tell us about it:
echo "Changing post number " . $thisPostHasIt . " to category number ". $my_category . "</p>";
}
echo "<p><b>All done!</b></p>";
?>