Well it all depends on how much you've changed...
In theory, if just the prefix to the tables were changed, then you won't need to change anything else, but if you've changed the names of fields, or the names to the tables, that would need to be reflected in any code that requires access to those tables.
// Initialize a User object.
public function __construct()
$this->uid = null;
$this->fields = array('username' => '', 'password' => '', 'email_address' => '', 'is_active' => false, 'permission' => 0);
I use slightly different naming conventions than the author of the book, so I had to change those in the user class.
public static function getById($user_id)
$user = new User();
$query = sprintf('SELECT username, password, email_address, is_active, permission FROM %suser WHERE user_id = %d', DB_TBL_PREFIX, $user_id);
$result = mysql_query($query, $GLOBALS['DB']);
$row = mysql_fetch_assoc($result);
$user->username = $row['username'];
$user->password = $row['password'];
$user->email_address = $row['email_address'];
$user->is_active = $row['is_active'];
$user->permission = $row['permission'];
$user->uid = $user_id;
The author uses uppercase letters for his field names, I don't... and these changes also need to be reflected in the user class.
Hope that helps to steer you in the right direction.