If you need to change mysql tables prefix just run the script below:

<?php
 $a = ''; // here goes old current prefix
 $b = ''; // here goes a new prefix

 $result = db_query("SHOW TABLES");
  while ($r = db_fetch_array($result)) {
   $table_old = current($r);
   $table_new = str_replace($a, $b, $table_old);
   db_query("RENAME TABLE {$table_old} TO {$table_new}");
   print "{$table_old} —> {$table_new} 
";
  }
?>

Of course you need to have mysql connection established at first.