Medicare open enrollment ends Dec. 7, but aspects of the sign-up period could extend into the new year for more than 300,000 people in Minnesota who are losing Medicare Cost plans.

The deadline details depend on whether consumers are replacing their Cost plans by reverting to original Medicare or buying a Medicare Advantage (MA) plan from a private insurer.

Cost plans are being eliminated next year across 66 counties in the state due to federal law. State officials say people leaving Cost plans can avoid surprise bills next year by enrolling in an MA plan with drug coverage or by supplementing original Medicare with a "Medigap" insurance policy plus a stand-alone Part D plan for prescriptions.

When it comes to Part D plans, now is the time for making a choice.

"If the affected Cost plan enrollee received their Part D plan from a separate stand-alone Part D plan, and want to stay with a separate Part D plan or enroll in a new Part D plan (and then purchase a separate Medigap), they need to enroll in a Part D plan by midnight on Dec. 7, 2018," the Minnesota Board on Aging said in a response to Star Tribune questions.

There's a bit more flexibility when it comes to Medigap plans. People losing Cost plans have a "guaranteed issue" right to buy a Medigap plan without answering questions about their health history through March 4. To have replacement coverage ready to go in January, the state says consumers going the Medigap route need to purchase the policy through an agent, broker or insurance company by midnight on Dec. 31.

There's even more flexibility when it comes to Medicare Advantage (MA), although the Dec. 7 deadline remains important for triggering later options.

If a Cost plan enrollee received their Part D benefits from their Cost plan, and they enter 2019 with enrollment in a Medicare Advantage plan, they can return to original Medicare in the new year and still use their guaranteed issue rights to a Medigap plan until March 4. These consumers also can purchase a stand-alone Part D plan until March 31.

This works like a safety net for Cost plan enrollees who are being automatically enrolled by their current insurer into a new MA plan. If they don't like the coverage, they'll have the chance early next year to get into original Medicare with supplementary coverage.

Separate from the Cost plan transition, Medicare beneficiaries who sign up for an Advantage plan by Dec. 7 will have the chance to switch to a different MA plan through a special enrollment period between Jan. 1 and March 31. This is a new option offered across the country by the federal government.