Add First Name & Last Name

You can use the get_user_by function to get that user data, in your case first name and last name… Example : add this in the functions.php and you can get the first and last name value to add in your email body!!

add_filter( "wp_username_changed_email_body", "your_function" );

function your_function( $old_username, $new_username )
{
    $email_body = "Your custom email text body.";
    $user = get_user_by( 'login', $new_username );
    
    if( $user )
    {
      $first_name=$user->first_name;
      $lastname = $user->last_name;
    }
    return $email_body;
}