Medicare open enrollment has been underway for a number of weeks now. During this time, existing Medicare enrollees can make changes to their coverage, such as swapping a Part D drug plan for another or switching Medicare Advantage plans.

If you're currently enrolled in Medicare Advantage, you may not want a new Advantage plan, though. Instead, you may be tempted to drop Advantage altogether and stick with original Medicare.

That's not necessarily a bad idea. When you get coverage under original Medicare, you generally get access to a much wider network of providers. And if you plan to split your time between two parts of the country (in other words, go the snowbird route), original Medicare could make a lot more sense, as you won't have to stress as much about finding doctors who are in-network.

A doctor standing over a patient lying in bed.

Image source: Getty Images.

But going from Medicare Advantage to original Medicare could mean incurring some added costs. Here are a few to anticipate.

1. A Part D drug plan

Medicare Advantage plans typically offer all-in-one coverage, so you don't need a separate drug plan. But once you move over to original Medicare, Parts A and B won't cover prescriptions, so you'll need to sign up for Part D. Now as is the case with Medicare Advantage, each Part D plan sets its own rules with regard to costs and coverage, so it's important to research your options wisely.

2. Higher deductibles and coinsurance

One benefit of Medicare Advantage is that these plans come with an out-of-pocket maximum you'll have to spend on healthcare. With original Medicare, there's no such maximum. That could prove problematic if your retirement income is largely limited to Social Security and you don't have a lot of wiggle room for unexpected medical costs.

Plus, depending on your coverage, moving over to original Medicare could result in higher deductibles and coinsurance. Throw in the services original Medicare won't cover, like dental cleanings and eye exams, and your costs have the potential to soar.

3. Supplemental insurance

If you're enrolled in Medicare Advantage, you can't get supplemental insurance known as Medigap. But you are able to sign up for a Medigap plan once you move over to original Medicare.

That Medigap plan, however, might cost you. Now the good news is that it can help cover some of your cost-sharing expenses to keep your deductibles and coinsurance costs down. But you should know that Medicap plans typically require you to go through medical underwriting, and there's no guarantee that you'll be able to get one.

You should also know that Medigap policies will only help cover your out-of-pocket expenses for service Medicare covers. If you wind up with a hospital stay, for example, your Medigap coverage might kick in to ease the burden of a high Part A deductible. But Medigap won't pay for a dental cleaning if that's something Medicare itself doesn't pay for.

There are plenty of good reasons to move from Medicare Advantage to original Medicare. Just be mindful of the costs you might incur if you end up going this route.