When rotating a Page, Annotation such as Textbox and Stamps remain in their original orientation (so they can be read without turning your head 90 degrees).
Rotating/Flipping a Page does not rotate any other Pages in the Plan Set but rotates every Revision of this Page. Each Page must be rotated separately.
If you realize that the Plan only must be rotated 90 degrees, you can return to the Plan Organizer and do so. This may cause takeoff or Annotations on the Page to fall "outside" the Page boundaries, making it impossible to see those objects. You can delete them, but only by using Ctrl+A and deleting everything on that Page. This is why the program prevents you from rotating the Plan independent of takeoff/annotations once it's been saved on the Takeoff Tab.