Medicare open enrollment starts soon. Here's what to know.

It's kick-off time for the 2027 annual Medicare open enrollment period for millions of seniors.

From Oct. 15 to Dec. 7, those who are enrolled can make changes to their coverage, which goes into effect on Jan. 1.

You can review the out-of-pocket costs you face in traditional Medicare, Medicare Advantage, and Medicare Part D — including premiums, deductibles, copays, and coinsurance.

And you have the opportunity to change from traditional Medicare to a Medicare Advantage plan or vice versa. You can also swap Medicare Advantage plans. And you can change your Part D prescription drug plan.

To find out what's changing for your 2027 Medicare or Medicare Advantage plan, scrutinize your Annual Notice of Change (ANOC) letter you should have received last month explaining the latest shifts.

If you ignored it, rescue it from the pile.

"There are changes beyond premiums underneath the hood, including cost-sharing and coverage of benefits, which means it's important to read the fine print," Juliette Cubanski, director of the program on Medicare policy at KFF, told Yahoo Finance.

About 7 in 10 people with Medicare don't comparison shop during the open enrollment period.

"Most people stick with the plan that they have from one year to the next unless the plan is withdrawn from the market, or there's a substantial premium change," she said. "Because of the complexity of the marketplace and of the decision making, when it comes to comparing all of these different plan options, many people feel overwhelmed and find it's easier just to sit it out."

What's changing in Medicare drug costs and coverage in 2027

Medicare Advantage is the alternative to traditional Medicare, with insurance plans offered through private companies. It covers more than half of all Medicare enrollees, or about 36 million people.

Average Medicare Advantage premiums are expected to decrease in 2027, but that is only one part of your overall costs.

"Even though average premiums are decreasing, you may have to pay a larger share of the costs when you do need medical care," said Kim Lankford, author of " Medicare 101 . " "Look carefully at the copayments and other out-of-pocket expenses for the type of medical care you frequently use, any procedures you expect to need in the upcoming year, and the plan's copayments for the first several days of a hospital stay."

Also make sure the doctors you want to use are covered, and find out how out-of-network providers are covered — Medicare Advantage PPOs usually provide some coverage out of network, but at higher copayments, while HMOs usually don't cover out-of-network providers, except for emergencies, Lankford said.