I want to add few comments to what @hopenr mentioned.
During the open enrollment plan, you can go from a Medicare Advantage plan to Original Medicare (Part A & B). But, majority of people also purchase a Medigap / Supplemental policy to cover the portion of medical expenses that Medicare does not cover. It is the Medigap policy that is not guaranteed if you do not sign up when become originally eligible for Medicare (usually at 65). It is called Guaranteed-issue rights and ensure that you can enroll in any Medigap plan sold in your state and that you won’t be charged higher premiums based on your health status.

When you switch from Medicare Advantage to Original Medicare, it’s usually the case that you lose your “guaranteed-issue” rights for Medigap. Without guaranteed-issue rights, your insurance company may require medical underwriting before it offers you a plan. During medical underwriting, the insurer looks at your past medical history and current health status. If the company determines the risk of covering you is too high, it can refuse to offer you the plan you want, or it may charge you much higher premiums for the coverage.

In summary, during open enrollment and considering changing your plan, you must look at all your options, coverage and expense.