It is possible to change the applications main form dynamically at runtime, but only during the applications start up. To do this, you will need to choose "View->Project Source" from the IDE's main menu, and edit the applications initialization code, changing the forms creation order in response to some condition. Note: You will need re-edit this code if you add additional forms to the project, as the IDE's code wizard will not be expecting the changes that you have made to the main project file, and you may find that the creation code for the additional forms you have added may have been inserted in an unexpected position within the code.

begin
  Application.Initialize;
  if SomeCondition then begin
    Application.CreateForm(TForm , Form );
    Application.CreateForm(TForm2, Form2);
  end else begin
    Application.CreateForm(TForm2, Form2);
    Application.CreateForm(TForm , Form );
  end;
  Application.Run;
end.