Healthcare is a major expense for seniors, and so it's important to do what you can to keep your costs reasonable. And part of that means making smart choices when it comes to Medicare.

This week, Medicare's open enrollment period will kick off on Oct. 15. And if it's your first go-round, here are a few key things you should know.

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1. It's for existing enrollees only

Seniors first become eligible to sign up for Medicare during a seven-month window that kicks off three months before the month of their 65th birthday and ends three months after that. If you're first looking to sign up for Medicare, then open enrollment isn't for you. Rather, it's for people who are already getting coverage through Medicare.

2. You have almost two months to make changes to your coverage

Medicare open enrollment begins on Oct. 15 and lasts all the way through Dec. 7. That gives you a nice window of time to review your plan choices and make changes accordingly. But you don't want to wait too long to start wading through your options. Comparing Medicare plans can be time-consuming, so the sooner you start the process, the less stressful it's apt to be.

3. You can change your Part D drug plan if there's a better one out there

If you're signed up for original Medicare, then you need a separate Part D plan for drug coverage. During open enrollment, you can move from one Part D plan to another if you're not satisfied with your existing coverage, or if your current plan has notified you of changes you aren't happy with.

4. You can change Advantage plans -- or move off of Advantage entirely

Some seniors opt for coverage under Medicare Advantage, an alternative to original Medicare. Just as you can change your Part D plan during open enrollment, you can also switch Advantage plans. And if you decide you're done with Medicare Advantage, you can move over to original Medicare and sign up for a Part D plan.

5. Your changes won't take effect right away

The plan choices you make during open enrollment will take effect at the start of 2023. If you decide to get a new Part D plan, for example, you won't be covered under that plan right away, so make sure to keep that in mind.

6. It pays to look at plan ratings

Medicare has a plan-finder tool on its site that allows you to explore your options for Part D and Advantage plans based on factors like your geographic location and prescription needs. You should also know that Medicare employs a star ratings system that can help you assess the quality of the different plan choices you're presented with. All plans are rated from one to five stars, with five being the best.

Make the most of open enrollment

Choosing the right Medicare coverage could help you stretch your retirement income further. And if your main source of income is your monthly Social Security check, then it's especially important to make savvy decisions during open enrollment. Now that you know what to expect, carve out some time to sort through your plan choices and set yourself up accordingly for 2023.