If you're thinking about dropping your Advantage Plan during open enrollment and instead relying on original Medicare, proceed with caution. Some retirees assume they'll be able to pair it with a Medigap policy, which helps cover the cost of deductibles, copays and coinsurance. Unless you meet a special exception when you make the switch, however, there's no guarantee you'll be extended Medigap coverage immediately, if at all. "We're encountering a lot of surprise that Medigap rules allow insurers to exclude coverage for pre-existing conditions," said Mary Johnson, Social Security and Medicare policy analyst for The Senior Citizens League. "There's a misconception that the 2010 [Affordable Care Act] made it so people would be guaranteed coverage no matter what. "That's not true for Medigap, and it can trip people up."

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When you first enroll in Medicare (typically at age 65), you get six months when you are guaranteed coverage through Medigap. That is, you can purchase a policy without an insurance company nosing through your health history and deciding whether to insure you. After that, it can be a different story. In most states, you'll have to go through medical underwriting. "They can impose waiting periods for pre-existing conditions, or deny coverage altogether," she said. Special exceptions to that include when your Advantage Plan is no longer available or when you are within the first year of enrolling in Medicare and decide to ditch your plan. While one of the options during the program's annual open enrollment period — which ends Dec. 7 — is to switch from an Advantage Plan to original Medicare (Part A hospital coverage and Part B outpatient services coverage), it's important to know its limitations.

For instance, original Medicare has no annual out-of-pocket maximum, unlike Advantage Plans. Additionally, both Parts A and B come with deductibles and limitations on what services are covered and to what degree. That's where Medigap policies help. Yet they aren't a cure-all: For instance, original Medicare does not cover dental and vision expenses, and Medigap policies do not pick up those costs. Advantage Plans, on the other hand, often offer coverage for both. Also, many Advantage Plans include prescription drug coverage (Part D). If you are on original Medicare — whether you have a Medigap policy or not — you must sign up for a standalone prescription plan unless you meet certain conditions. Additionally, depending on a combination of factors — the Medigap policy's features, where you live, and sometimes your age — the cost can reach a few hundred dollars a month.