You need to change the printer, driver, and port as listed in the device key of the Windows section in the Windows.ini file, then broadcast a system wide WM_WININICHANGE message. Example:

procedure TForm .Button Click(Sender: TObject);
var
  WinIni : TIniFile;
  WinIniFileName : array[0..MAX_PATH] of char;
  s : array[0..64] of char;
begin
  GetWindowsDirectory(WinIniFileName, sizeof(WinIniFileName));
  StrCat(WinIniFileName, '\win.ini');
  WinIni := TIniFile.Create(WinIniFileName);
  try 
    WinIni.WriteString('windows',
                       'device',
                       'HP LaserJet Series II,HPPCL,LPT :');
  finally
    WinIni.Free;
  end;
  StrCopy(S, 'windows');
  SendMessage(HWND_BROADCAST, WM_WININICHANGE, 0, LongInt(@S));
end;