Medicare is a vital part of retirement planning for millions of Americans. Healthcare costs are so high, it's important for everyone to figure out how to make the most of their Medicare benefits. That's why it's essential to take a look at your healthcare needs each year and consider making changes to your Medicare coverage under the annual open enrollment period that runs from mid-October to early December.

With open enrollment still months away, you might not want to wait to make coverage changes. There are several situations in which you don't have to wait for open enrollment to make decisions about Medicare that can improve the coverage you get. By taking advantage of these situations when they come up, you sometimes can save a lot of money compared to what you'd pay if you had to wait before making changes.

You just turned 65

Most people become eligible for Medicare when they turn 65, and you don't have to wait until that year's open enrollment to sign up. Your initial enrollment period begins three months before you turn 65 and ends three months after your 65th birthday, and if you enroll early, your coverage will start as soon as possible once you're eligible.

Initial enrollment gives you all the choices you have during open enrollment, including choosing among traditional Medicare, a Medicare Advantage plan, a Medigap supplemental policy, and Part D prescription drug coverage. For some, enrollment is automatic, but if you don't get a Medicare card in the mail three months before you turn 65, you'll probably need to enroll on your own.

A Medicare enrollment form with a pen on top of it.

Image source: Getty Images.

You're eligible for both Medicare and Medicaid

Those who get additional assistance from their state Medicaid programs have more flexibility than those who only receive Medicare. A special enrollment period for dual participants allows them to join, switch, or drop a Medicare Advantage Plan or a prescription drug coverage plan at any time.

You lose Medicaid coverage

If you had Medicaid coverage but then became no longer eligible, you have two full months after you find out to make changes. You can join a Medicare Advantage plan or prescription drug plan, switch among plans, drop a prescription drug plan, or drop Medicare Advantage and return to original Medicare.

You move to a new area outside your current plan's service area

If you're on a Medicare Advantage plan or prescription drug plan and you move beyond the area it serves, you can pick a new one. You can also choose to return to original Medicare. If you inform the plan of your move, then you can switch up to a month before you move or two more months afterward. If you wait until after you move to inform your plan, you can switch as soon as you tell your plan you've moved or up to two months later.

Your coverage from an employer group plan changes

Some people over 65 still have health coverage under an employer group plan. If that eligibility ends, you can immediately sign up for Medicare under a special enrollment period that lasts for two months after your coverage ends. Conversely, if you've been covered under Medicare and get a job that offers coverage, you can drop Medicare Advantage or prescription drug plans whenever your employer lets you join the private group plan.

Other situations

There's a wide range of other instances in which you can make changes to Medicare outside open enrollment. Moving back to the U.S. from living abroad, getting released from jail, or losing coverage under a variety of different assistance programs at the state and federal level can trigger immediate rights to join, drop, or make changes to your Medicare coverage. You also can make one switch to a five-star Medicare Advantage or prescription drug plan during each calendar year between Dec. 8 and the following Nov. 30. For a complete list of situations, this Medicare website is a good place to start.

Healthcare is important as you get older, and making sure your Medicare works as hard as it can for you will help you be more financially secure, as well as keep you healthy. By knowing when you can make Medicare moves, you'll be less likely to put up with subpar options and be able to move more aggressively to get exactly the healthcare coverage you need.