Medicare enrollment is one of the most consequential financial decisions retirees make — and one of the most misunderstood. From choosing between Original Medicare and Medicare Advantage to understanding how income affects premiums, the decisions made at enrollment can shape coverage quality and out-of-pocket costs for years. To cut through the confusion, Investopedia consulted five experts — including certified financial planners and an attorney specializing in Medicare — about the most common and costly mistakes people make when enrolling or choosing a plan.
Why Medicare Enrollment Demands More Research
Medicare is not a single product. It is a system of interconnected coverage options with distinct rules, restrictions and cost structures. Original Medicare — Parts A and B — provides broad provider access but carries no out-of-pocket maximum and requires supplemental coverage to manage costs. Medicare Advantage — Part C — bundles coverage through private insurers, caps annual spending but limits provider choice. Each path carries trade-offs that only become visible through deliberate comparison. Many enrollees skip this comparison entirely, defaulting to whatever option feels familiar or was recommended casually by a friend. That shortcut frequently leads to regret.
Mistake 1: Not Comparing Original Medicare and Medicare Advantage
The Network Trap Many Enrollees Miss
Jason Rubin, an independent insurance agent specializing in Medicare, identifies insufficient research as the most common enrollment error. In 2025, more than 54% of eligible Medicare beneficiaries chose Medicare Advantage. Yet many did so without fully understanding what that choice entails. When enrolling in a Medicare Advantage plan, beneficiaries assign their Medicare Part A and Part B benefits to a private insurance company. That company then controls where they can receive care. Unlike Original Medicare — which allows beneficiaries to see any provider who accepts Medicare — Medicare Advantage plans are network-based. All physicians, including primary care providers and specialists, must belong to the same medical network. Furthermore, many Medicare Advantage plans require prior authorization for care and referrals before seeing specialists. Enrollees with multiple existing specialists are particularly vulnerable to disruption if those providers do not all participate in the same network.
Mistake 2: Assuming You Can Always Switch Plans
The Medigap Window That Closes Only Once
David Lipschutz, an attorney and co-director of the Center for Medicare Advocacy, identifies a widespread and costly misconception: many people believe they can freely move between Medicare Advantage and Original Medicare at any time. Technically, switching during open enrollment periods is possible. However, the practical obstacle lies with Medigap — the supplemental policies that cover costs not paid by Original Medicare. In 2022, 42% of people on Original Medicare carried a Medigap policy. The critical detail most enrollees miss is that Medigap insurers have a one-time guaranteed issue window — a six-month period beginning when a beneficiary first gets Medicare Part B. During that window, insurers cannot deny coverage or charge higher premiums based on health status. Outside that window, Medigap insurers can and routinely do deny coverage based on pre-existing conditions. Lipschutz is direct: many people enroll in Medicare Advantage thinking they can switch to Original Medicare with a Medigap policy two years later. In most cases, that assumption is wrong.
Mistake 3: Not Reviewing Your Plan Every Year
Brandy Bauer, joint center director of the Northeast Iowa Agency on Aging, highlights plan inertia as a pervasive and expensive habit among Medicare beneficiaries. Every autumn, Medicare’s open enrollment period gives Medicare Advantage enrollees and standalone prescription drug plan holders the opportunity to reassess their coverage and switch plans. The vast majority do not take advantage of this window. Yet plan benefits, drug formularies and provider networks change annually. A plan that offered strong value one year may charge higher premiums, cover fewer drugs or narrow its provider network the next. Consequently, beneficiaries who stay in the same plan year after year frequently overpay or lose coverage quality without realizing it. Reviewing plan options each fall during the October 15 to December 7 enrollment window is one of the most straightforward ways to protect both coverage and cost.
Mistake 4: Ignoring How Income Affects Medicare Premiums
IRMAA: The Premium Surcharge Many Enrollees Never See Coming
Patrick Huey, a certified financial planner and owner of Victory Independent Planning, identifies a different category of mistake entirely — one that has nothing to do with choosing between plan types. His number one concern is the Income-Related Monthly Adjustment Amount, known as IRMAA. Medicare Part B and Part D premiums are not flat rates for all enrollees. They are income-dependent, calculated based on the modified adjusted gross income reported on tax returns from two years before the enrollment year. The standard 2025 Part B premium is $185 per month. However, high earners pay substantially more. Married couples with a MAGI above $266,000 and up to $334,000 in the relevant tax year pay $370 per month for Part B — double the standard rate — before adding any Part D premium surcharges.
How Financial Moves Can Trigger IRMAA Surcharges
Huey warns that seemingly smart financial moves made in the years before Medicare enrollment can unintentionally trigger IRMAA surcharges. Large Roth conversions, IRA withdrawals, capital gains realizations and home sales can all push MAGI above IRMAA thresholds — permanently raising Medicare premium costs for that enrollment year. Mark Stancato, CFP and founder of VIP Wealth Advisors, addresses this directly with clients by running every Roth conversion through a multi-year projection that weighs future tax savings against the near-term Medicare cost impact. Without that analysis, he notes, what appears to be a smart tax move can backfire quickly. Pre-retirees should therefore coordinate Medicare enrollment planning with their tax and financial advisors — not treat them as separate decisions.
What Experts Say You Should Do Instead
The collective guidance from all five experts points toward the same core principles. First, research Medicare Advantage and Original Medicare thoroughly before enrolling — including network restrictions, prior authorization requirements and long-term switching limitations. Second, understand the Medigap enrollment window and protect it. Missing that six-month guaranteed issue period can permanently limit supplemental coverage options. Third, review plan coverage every year during open enrollment rather than defaulting to the previous year’s plan. Fourth and finally, plan Medicare premium costs as part of a broader tax and financial strategy — particularly in the years before enrollment when large income events can trigger lasting IRMAA surcharges. Medicare decisions made carefully protect both health access and financial stability well into retirement.
