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;