Medicare open enrollment, which runs from Oct. 15 to Dec. 7 each year, is officially in full swing. That means you now have an opportunity to make changes to your existing Medicare coverage.

One change you may be interested in is switching from original Medicare (Parts A and B plus a Part D drug plan) to a Medicare Advantage plan. Doing so could set you up with superior coverage and allow you to save on healthcare costs. But if you're not quite sure how Medicare Advantage plans work, here are some key things to know.

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1. They have to cover all the services original Medicare covers

The good thing about signing up for a Medicare Advantage plan is that you're guaranteed to get at least the same level of coverage you'd get under original Medicare. For some people, the lure of an Advantage plan can be cost savings. But you shouldn't lose out on coverage for essential services in the course of saving that money.

2. They commonly cover additional services

Original Medicare generally does not pay for dental care, eye exams, and hearing aids, all of which can be fairly common expenses for seniors. The upside of Medicare Advantage plans is that they commonly offer supplemental benefits on top of what original Medicare provides.

In fact, most Advantage plans include dental coverage, and many of these plans cover eye exams and hearing aids as well. You may even get benefits on top of those that include paid gym memberships and meal delivery services.

3. They can be more cost-effective

To enroll in a Medicare Advantage plan, you actually have to pay a premium for Part B and then, in some cases, another premium for your Advantage coverage itself. But your total costs under Medicare Advantage can still be lower than your total costs under original Medicare.

Some Advantage plans, in fact, have a $0 premium. It's important to review your Medicare Advantage plan choices, as each plan in your area has its own specific benefits and costs.

4. They can be more limiting

When you get coverage with original Medicare, you can see any provider that accepts Medicare. With Medicare Advantage, you're generally limited to a specific network of providers. That could make it more difficult to get the care you need.

Plus, provider networks can change from one year to the next. So even if you find trusted providers within your plan, you never know when they might leave that network.

5. Plan rules and benefits can change annually

We just learned that providers can go from being in-network to not in-network with a Medicare Advantage plan. But that's not the only change your plan might undergo.

Medicare Advantage plans can change annually in terms of costs and benefits. So once you sign up for one, it's important to review your plan's changes every year. The good news is that you can always switch to a new plan during open enrollment.

If you've never really looked into Medicare Advantage, it may be that you don't know what you're missing. Or, it may be that original Medicare is a better choice for you. But now that open enrollment is upon us, it's a good idea to explore your options for an Advantage plan and see if it pays to make a switch.