Introduction to Medicare Enrollment Periods
Understanding Medicare’s enrollment windows is critical to making informed choices about your healthcare coverage. Two of the most talked-about periods are the Annual Enrollment Period (AEP) and the Medicare Advantage Open Enrollment Period (OEP).
While they sound similar, they serve very different purposes—and knowing the difference could save you money and stress.
What Is the Annual Enrollment Period (AEP)?
When AEP Happens
The Annual Enrollment Period runs from October 15 to December 7 each year. Coverage changes made during this window become effective on January 1 of the following year.
Who Can Use It?
All Medicare beneficiaries—regardless of whether they have Original Medicare or a Medicare Advantage plan—can use AEP to make changes.
What You Can Do During AEP
- Enroll in a Medicare Advantage plan
- Switch from one Medicare Advantage plan to another
- Drop Medicare Advantage and go back to Original Medicare
- Enroll in or switch a Part D prescription drug plan
- Drop drug coverage completely
There’s no limit to the number of changes you can make during AEP—the last selection submitted by December 7 will take effect January 1.
What Is the Open Enrollment Period (OEP)?
When OEP Happens
The Open Enrollment Period for Medicare Advantage runs January 1 to March 31 each year.
Who Qualifies
Only those already enrolled in a Medicare Advantage plan as of January 1 are eligible to use OEP.
What You Can Change During OEP
- Switch from one Medicare Advantage plan to another
- Drop your Medicare Advantage plan and return to Original Medicare
- Add a standalone Part D drug plan if returning to Original Medicare
However, you can’t enroll in Medicare Advantage for the first time or switch Part D plans unless returning to Original Medicare.
Side-by-Side Comparison of AEP vs OEP
| Feature | Annual Enrollment (AEP) | Open Enrollment (OEP) |
| When | Oct 15 – Dec 7 | Jan 1 – Mar 31 |
| Who Can Use It | All Medicare beneficiaries | Current Medicare Advantage enrollees |
| Switch to MA Plan | Yes | No |
| Switch MA Plans | Yes | Yes |
| Return to Original Medicare | Yes | Yes |
| Part D Plan Changes | Yes | Only when returning to Original Medicare |
| Changes Allowed | Multiple | One-time only |
Eligibility Rules for AEP and OEP
AEP Eligibility Explained
All individuals who are enrolled in Medicare—whether through Original Medicare or a Medicare Advantage plan—are eligible to make changes during the Annual Enrollment Period (AEP).
There are no restrictions based on prior plan choices. As long as you’re enrolled in any form of Medicare, you can:
- Join or drop a Medicare Advantage plan
- Switch from Original Medicare to an Advantage plan, or vice versa
- Enroll in or change your prescription drug (Part D) plan
OEP Eligibility Explained
The Open Enrollment Period (OEP) is limited to those who are already enrolled in a Medicare Advantage plan as of January 1. If you are not in a Medicare Advantage plan on that date, you cannot use OEP to make any changes.
Only one change is allowed during OEP, so it’s critical to make the right decision the first time.
Why Timing Your Plan Changes Matters
Enrollment Deadlines
Missing AEP means you’re locked into your current plan for another year—unless you qualify for a Special Enrollment Period (SEP). Likewise, missing OEP can leave you with a plan that doesn’t meet your needs until the next enrollment opportunity.
Effective Dates of Changes
- Changes during AEP go into effect on January 1
- Changes during OEP take effect on the first day of the month following the plan change
Risks of Missing Key Periods
- You may lose access to your doctors or prescription coverage
- Premiums and out-of-pocket costs could increase unexpectedly
- You might be stuck with benefits that no longer meet your needs
How to Use AEP to Your Advantage
The Annual Enrollment Period is your best opportunity to fully reassess and optimize your Medicare coverage.
Switching Plans
You can switch from:
- Original Medicare to a Medicare Advantage plan
- One Medicare Advantage plan to another
- One Part D drug plan to another
Adding/Removing Drug Coverage
AEP allows you to:
- Enroll in a standalone Part D plan if you’re on Original Medicare
- Drop drug coverage if you no longer need it (but be cautious—late enrollment penalties can apply later)
Returning to Original Medicare
If your Medicare Advantage plan isn’t meeting your needs, you can switch back to Original Medicare and optionally add a Medigap policy and standalone drug plan.
Maximizing OEP for Better Medicare Advantage Coverage
Making a One-Time Change
During OEP, you can:
- Switch Medicare Advantage plans if your current one no longer works for your health needs or budget
- Return to Original Medicare if you find Medicare Advantage isn’t the right fit
Fixing Plan Mistakes
OEP is ideal if you realize after AEP that:
- Your provider is no longer in-network
- Your prescription drug is no longer covered
- You didn’t understand the costs or coverage details of the plan you selected
Returning to Original Medicare
If you switch back to Original Medicare during OEP, you can also enroll in a standalone Part D plan, though Medigap policies may require underwriting unless you’re within your trial rights period.
Prescription Drug Plan Changes: What’s Allowed?
Part D Changes in AEP vs OEP
| Action | AEP | OEP |
| Add Part D | Yes | Only if returning to Original Medicare |
| Drop Part D | Yes | No (unless dropping MA-PD and returning to Original Medicare) |
| Switch Part D | Yes | No |
AEP is the only time most beneficiaries can freely switch drug plans. OEP restricts drug plan changes unless you are moving away from a Medicare Advantage plan entirely.
Mistakes to Avoid During Enrollment Periods
- Assuming You Can Switch Anytime
Outside of AEP and OEP, plan changes are rare unless you qualify for an SEP. - Overlooking Plan Details
Make sure to read the full summary of benefits—don’t just focus on premiums. - Missing Provider or Network Changes
Double-check if your doctors and pharmacies are still covered in the plan for next year. - Waiting Too Long to Decide
The final days of AEP can be stressful—plan ahead to avoid last-minute decisions.
Special Enrollment Periods (SEP): The Exception Rules
If you miss AEP or OEP, Special Enrollment Periods (SEPs) may still allow you to make changes if you experience:
- A move outside your plan’s service area
- Loss of employer or union coverage
- Eligibility for Medicaid or Extra Help
- Entry into or exit from a skilled nursing facility or long-term care facility
Each SEP has a different duration and rules—act quickly when a qualifying event occurs.
Smart Strategies for Comparing Medicare Plans
Use these strategies to make the best choice:
Use Medicare.gov Tools
Visit Medicare Plan Finder to compare plans based on cost, coverage, provider networks, and drug formularies.
Key Questions to Ask
- Are my preferred doctors in-network?
- Are my prescriptions covered?
- What are the out-of-pocket maximums?
- What’s the total cost, not just the premium?
Consider Costs, Coverage, and Networks
Don’t be swayed by low premiums alone. Total costs include:
- Deductibles
- Copayments
- Coinsurance
- Maximum out-of-pocket limits
FAQs About AEP vs OEP
Q1: Can I switch from Original Medicare to Medicare Advantage during OEP?
No. This switch is only allowed during AEP or with a Special Enrollment Period.
Q2: What if I change my mind after making a switch in OEP?
You can only make one change during OEP. After that, you’re locked in for the year unless you qualify for an SEP.
Q3: Can I enroll in a Part D plan during OEP?
Only if you’re returning to Original Medicare from a Medicare Advantage plan.
Q4: Do changes made during AEP or OEP take effect immediately?
No. AEP changes start January 1, and OEP changes take effect the first of the month after your request is processed.
Q5: Can I drop my Medicare Advantage plan at any time?
No. Plan changes are only allowed during AEP, OEP, or with a SEP.
Q6: What happens if I miss both AEP and OEP?
You’re generally locked into your current plan for the year unless you qualify for an SEP.
Final Thoughts: Make the Right Move at the Right Time
Understanding the differences between AEP and OEP empowers you to make smart decisions about your Medicare coverage. Whether you want to explore new benefits, lower your costs, or fix an enrollment mistake, timing is everything.
✅ Use AEP (Oct 15 – Dec 7) to explore all options.
✅ Use OEP (Jan 1 – Mar 31) if you’re already in Medicare Advantage and need to adjust.
✅ Use SEP if life circumstances allow.