Just as millions of seniors routinely rely on Social Security, so too is Medicare an extremely important program for older Americans. And with fall open enrollment right around the corner, it's important that seniors take advantage of this crucial seven-week period. Here are five things to know about Medicare open enrollment.

1. It's only for current enrollees

If you're ready to sign up for Medicare, great -- you can enroll in benefits starting up to three months before the month of your 65th birthday. But if you're not on Medicare yet, there's nothing for you to do during open enrollment. Open enrollment is only for seniors who are already receiving health coverage through Medicare, not for those signing up for the first time.

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If you need to complete your initial enrollment, you can do so by creating an account on the Social Security Administration's website. Keep in mind that you do not have to sign up for Social Security benefits if you only want Medicare coverage. Similarly, you'll have the option to elect coverage for Part A only, which covers hospital care, if you're not ready to sign up for Part B, which covers outpatient care.

2. You can change your coverage in time for 2022

If you're unhappy with your Medicare coverage and want a different set of benefits in place to kick off 2022, you'll need to make changes during open enrollment. While Medicare Advantage has a special open enrollment period that starts at the beginning of the calendar year, if you're on original Medicare and have a Part D drug plan, you'll want to take action during the fall.

3. You can switch Medicare Advantage plans

Even though Medicare Advantage has its own special open enrollment period that runs from January through March, you can still switch Advantage plans during open enrollment in the fall. That way, you'll have new coverage all set up for the new year.

4. You can get a new Part D drug plan

If you're on original Medicare, you'll need prescription drug coverage -- and now's your opportunity to make changes to your Part D plan. Even if you're happy with your existing drug coverage, it never hurts to see whether there's a better plan out there for you. You may find one that's more cost-effective.

5. You can go back to original Medicare

Many seniors wind up quite happy with Medicare Advantage. But if you're not thrilled with your existing plan and also aren't having luck finding a new one that suits your needs, you're not stuck. You can always revert to original Medicare. Just remember to sign up for a Part D drug plan for complete coverage.

The fact that Medicare offers seniors choices is a good thing, as it means you can customize your coverage based on your specific healthcare needs. Open enrollment begins on October 15 and it runs through December 7. But don't wait too long to review your coverage options. You may run into questions in the course of your research, so the sooner you get started, the better.