[__cadetill]

Sólo has de cambiar el Step del TProgressBar a -1

procedure TForm1.Button1Click(Sender: TObject);
var
  i : integer;
begin
  ProgressBar1.Position := 0;
  ProgressBar1.Step := 1;
  ProgressBar1.Max := 100;
  ProgressBar1.Min := 0;
  for i := ProgressBar1.Min to ProgressBar1.Max - 1 do
  begin
    Label1.Caption := IntToStr(ProgressBar1.Position);
    ProgressBar1.StepIt;
    Sleep(100); // solo es para ver el visionado
    Application.ProcessMessages;
  end;
  ProgressBar1.Step := -1;
  for i := ProgressBar1.Min to ProgressBar1.Max - 1 do
  begin
    Label1.Caption := IntToStr(ProgressBar1.Position);
    ProgressBar1.StepIt;
    Sleep(100); // solo es para ver el visionado
    Application.ProcessMessages;
  end;
end;