You cannot change printer property during printing of the document. But you can interrupt print process to change properties and run print again from the necessary line.

uses Printers;
...
procedure TForm1.Button1Click(Sender: TObject);
var
  F, F2: TextFile;
  i, j: Integer;
begin
  AssignPrn(F);
  Rewrite(F);
  Writeln(F, RichEdit1.Lines[0]);
  for i:=1 to RichEdit1.Lines.Count-1 do
  begin
    if Printer.PageNumber1 then
    begin
      CloseFile(F);
      Break;
    end;
  end;

  AssignPrn(F2);
  Printer.Orientation:=poLandscape;
  Rewrite(F2);
  for j:=i to RichEdit1.Lines.Count-1 do
    Writeln(F2, RichEdit1.Lines[j]);
  CloseFile(F2);
end;