Once you enroll in Medicare, you can choose a specific Medicare Advantage or Part D drug plan based on your needs. But the Medicare Advantage or Part D plan you start out with isn't necessarily the plan you should keep forever.

Medicare plans can change from one year to the next. And if yours is changing for the worse, that's something you need to know about. There's a key document that summarizes plan changes every year, and it's definitely something you should pay attention to.

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Keep an eye out for your Medicare plan's annual notice of change

Each fall, all Medicare plan administrators are required to send enrollees an annual notice of change. That notice should arrive no later than Sept. 30. So if you don't receive it by then, make sure to reach out to your plan administrator to follow up.

Your plan's notice of change, as the name implies, is meant to loop you in on changes to your coverage that are set to take effect in the new year. Those may include:

• Changes to your plan premiums, copays, or deductibles
• Changes to your Medicare Advantage plan benefits, such as added benefits or discontinued benefits
• Changes to prescription drug formularies, which could impact your costs
• Changes to your provider network, such as whether certain doctors and specialists are moving into or out of your plan's network

It's important to read your notice of change thoroughly so you understand what's happening with your Medicare coverage. And that's something you don't want to delay.

Why you need to read your notice of change immediately

The reason you should review your plan's notice of change immediately is simple. It's important to know what you're looking at ahead of Medicare's fall open enrollment.

Medicare open enrollment begins every year on Oct. 15 and runs until Dec. 7. During this time, you can make changes to your Medicare coverage, whether it's switching from one Part D plan to another, swapping Medicare Advantage plans, or ditching Medicare Advantage completely in favor of original Medicare.

If you don't know what's happening with your current plan, it'll be harder to determine if a new plan is better for you. So once that notice of change arrives, carve out some time to read it carefully. And if anything is unclear, reach out to your plan administrator to get clarity. That should allow you to approach open enrollment with the information you need to make an informed decision.