If i have AARP Medicare complete Choice PPO now, but in few years

I want to change to Plan F, can I do it or do I need to be qualified ( not to sick) to do so? Is it any plan in the AARP United Health Care Suplemet
( Plan F, N etc ) that I have the guaranty that I can change independent of the health issues that I will have at the time?
Thank you very much
Gloria Negrao