Medicare Advantage and Medicare Supplement insurance, also called Medigap, are types of private health insurance that help cover health care expenses in retirement. The former replaces your Original Medicare (parts A and B) coverage; the latter supplements your Medicare coverage with benefits that help you pay out-of-pocket health costs.

However, you can't be enrolled in both programs at the same time. If you're considering a switch from Medicare Advantage to Medigap, or vice versa, here's what you need to know.

What's the Difference Between Medicare Advantage and Medigap?

Medicare Advantage plans, also called Medicare Part C, are offered by Medicare-approved private companies. They include Medicare parts A and B, and usually drug coverage (Medicare Part D), too — effectively letting policyholders get all their Medicare insurance through a private insurer.

Medigap is a private health insurance plan that helps policyholders fill in the financial gaps in their Medicare coverage. Medigap policies might pay for some costs that Medicare doesn't cover, such as deductibles, co-payments and coinsurance.

When Can I Switch From Medicare Advantage to Medigap?

You cannot be enrolled in a Medicare Advantage and a Medigap policy at the same time. If you have a Medicare Advantage plan and want to switch to a Medigap plan, you'll have to drop your Advantage plan and re-enroll in Original Medicare. However, there are only certain time frames during which you can do that:

• Medigap open enrollment: You can purchase a Medigap policy during the six-month open enrollment period after you turn 65 and enroll in Medicare Part B. If you don't enroll during this initial open enrollment period, you might end up paying pricier premiums.
• Medicare Advantage open enrollment: There's an open enrollment period for Medicare Advantage every year between Jan. 1 and March 31. You can switch to a different Medicare Advantage plan during that period, or you could leave it and return to Original Medicare.
• Medicare open enrollment: From Oct. 15 to Dec. 7 every year, you can enroll in or drop any Medicare health plan.
• A special enrollment period: Not all Medicare Advantage plans are offered in all areas; if you move out of your Medicare Advantage benefit area, you might be able to switch from Medicare Advantage to Medigap.
• Medicare Advantage trial period: People who enroll in Medicare Advantage plans when they're first eligible (or who leave a Medigap plan to enroll in Medicare Advantage for the first time) get a 12-month trial period. If you leave Medicare Advantage during the trial period, you have a guaranteed right to purchase a Medigap policy.

Although you have the right to switch from Medicare Advantage to Medigap during the times above, there might be some costs involved in the switch. Medigap usually follows medical underwriting standards, meaning that insurance companies will examine your health information and medical history when evaluating you for coverage. You could be denied a Medigap policy or charged higher premiums if you have certain preexisting conditions or health risks.

However, if you purchase a Medigap policy when you are initially eligible for Medicare (when you turn 65 and enroll in Medicare Part B), you don't have to undergo medical underwriting. During the initial enrollment period, Medigap policies are guaranteed-issue, which means you cannot be denied a policy or charged higher premiums because of your health. The only other time frame during which you can count on a guaranteed-issue Medigap plan is if you purchase a policy after your Medicare Advantage trial period. Outside of those two time frames, you run the risk of paying higher premiums or being denied Medigap coverage when you switch coverage plans.

What's Right for Me?

Understanding your Medicare options can help you make the best decision for your health and wallet. Medicare Advantage and Medigap can help you save money on your health care expenses, but the right plan will depend on your health, financial needs and timing. If you're considering a switch from Medicare Advantage to Medigap, or vice versa, speak with an insurance agent or Medicare representative, who can give you the confidence you need to make a decision.