The open enrollment window for Medicare open enrollment is open until March 31, allowing retirees to switch back to original Medicare from Medicare Advantage plans to switch plans during this period guaranteed under federal Medicare enrollment rules. However, this change is often overlooked by retirees who mistakenly miss the enrollment deadlines. The real risk lies in a separate type of insurance, Medigap, which most people assume they can buy whenever they want. Medicap insurers can refuse to sell you a policy if your medical history or current health conditions make you too expensive to insure. This process, known as medical underwriting, applies in most states once your initial MedigAP enrollment window closes. However there are limited exceptions where you can still get Medigapped without medical underwritten. If you join Medicare Advantage when first eligible at age 65, you can switch back from Medicare within your first year.