If you're a senior on Medicare — or an adult child responsible for a senior on Medicare — here's something you should know: The annual "open enrollment" period for joining or changing prescription drug or private health plans is already under way.

"It's much earlier this year. It started on Oct. 15, and it's going to stop on Dec. 7," says Nancy Metcalf, a senior editor and health expert at Consumer Reports. "So you have your window right now."

Metcalf says the dates were moved up in part so plans could get all their paperwork done — and put new membership cards in patients' hands — by Jan. 1, when plan changes take effect.

But the new dates are only one of several changes Metcalf says Medicare enrollees need to be aware of for next year. Consumer Reports has a free guide to some of them.

A key change she says consumers should be aware of during the open season is a new "star-based" quality rating, particularly for health plans.

"You want 4 to 5 stars," Metcalf says. And the reason that matters is that "those plans are getting more money from Medicare. They're basically getting bonuses for being good."

And more money, in this case, means more benefits, she says, because "one of the deals is the extra money has to go toward benefits to the members, not into the pockets of the plans."

But no matter how many stars a plan has, last year's health law is guaranteeing certain new benefits to everyone enrolled in the private Medicare plans known as Medicare Advantage.

In 2011, everyone in traditional Medicare was guaranteed a series of free preventive benefits, including an annual wellness exam and screenings for things like cancer and heart disease. Starting in 2012, those benefits will have to be offered by Medicare Advantage plans, as well.

Next year will also see a further constriction of the notorious "doughnut hole" gap in Medicare prescription drug coverage, where people continue to pay premiums but essentially get no benefits. In 2012, people who reach the gap will automatically get a 50 percent discount on their brand-name prescription drugs, and a 14 percent discount on generic medications.

Metcalf says even if you spend a lot on drugs and will hit that gap, the Medicare plan finder website can help you budget. If you enter both the drugs you take and the drugstores you plan to use, she says, "you get this amazing spreadsheet that will tell you month by month how much out of pocket you're going to pay, and then exactly when you're going to hit the doughnut hole."

By 2020 the gap is scheduled to phase out altogether.

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