You can use regex:

ON $*:Notice:/(illegal|currently|changed)/iS:*: {
  if ( $nick == OperServ ) {
    if ($regml(1) == illegal) {
      notice %nick The %newnick Have Illegal Charters
    }
    elseif ($regml(1) == currently) {
      notice %nick The %newnick Is Currently In Use
    }
    else {
      notice %newnick Your Nick Has Been Changed To %newnick
    }
  }
}

This will tell the notice event to listen to those three words if found, rather than use a general notice to listen to all notices via the if checks.