Actually, there's a catch. If you look in the Federal PFFS Guide, it says that you have a 12 month trial in which you can return to your original Med Supp, however, if you didn't have a Med Supp to begin with you are stuck. Can't move until Open Enrollment (sorry for the bad news, but I learned this the hard way.) I have a copy of the guide that I will share if anyone wants it.
That is exactly how I understand it and have been taught. They have to go back to their original Med Sup plan and company. You hit the nail on the head (I mean head on the nail) with the fact that they can not go to a Med Sup GI if they first signed up with an MA and did not have a Med Sup. If in doubt you should sign them up with a Med Sup when they age in at IEP.