Medicare's annual Open Enrollment Period is in its final five weeks. It opened on October 15 and closes on Monday, December 7, and every change a beneficiary makes during it takes effect on January 1, 2027. A patient who has been on Original Medicare with a supplement for ten years can join a Medicare Advantage plan with a single phone call in late November, and the first your practice hears of it is a denial in the second week of January that says the patient is not eligible with the payer you billed.
This happens every year and it is entirely predictable. The scale is what people underestimate. Between the Advantage plans marketing hard through December 7 and the plan exits and benefit changes that push people to shop, a meaningful share of a practice's Medicare panel can be on a different plan on January 2 than on December 31. We wrote in October about what to prepare when the period opened. This piece is about the last weeks and the January work, because that is where the denials are made.
Key takeaways
- Every change made by December 7 takes effect January 1, 2027, and a second switching window for Advantage members runs January 1 to March 31.
- Treat every Medicare patient on the January schedule as new: batch eligibility in the last week of December and again 48 hours before the visit.
- Standing authorizations do not transfer between plans; list every one that crosses December 31 and re-verify it under the new plan.
- Decide before January how the practice handles members of Advantage plans you do not participate with.
- Ask each Advantage plan you work with for a disenrollment list in early December and again in early January.
What is actually changing on January 1
During Open Enrollment, a beneficiary can move from Original Medicare to a Medicare Advantage plan, from Advantage back to Original Medicare, from one Advantage plan to another, and can join, switch or drop a Part D prescription drug plan. Medicare's own guidance is that the plan must receive the request by December 7 for coverage to start January 1. There is a second window, the Medicare Advantage Open Enrollment Period from January 1 to March 31, during which someone already in an Advantage plan can switch once more or return to Original Medicare. So the churn does not end on New Year's Day; it continues through the first quarter.
For the practice, each move changes at least one of the following: who you bill, whether you are in network, whether a referral or prior authorization is required, what the copay is, and which plan the patient's drugs run through. A patient who moves from Original Medicare to an Advantage plan you do not participate with becomes an out-of-network visit overnight. A patient who moves the other way, from Advantage to Original Medicare, may have bought a new Medigap policy, or may not have been able to (the right to buy a supplement without medical underwriting is limited outside certain windows), which changes what the patient owes at the visit and how the secondary claim is filed.
How big the problem is for a typical panel
Put numbers on it before deciding how much effort to spend. Suppose a primary care practice has 1,200 patients over 65, of whom about 55 percent are in Advantage plans and 45 percent in Original Medicare. If eight percent of the panel changes something during Open Enrollment (a figure we use for planning; your payer mix and local market decide the real one), that is roughly 96 patients with a different payer, member ID or plan on January 1. If half of them have a visit in January and none of them are caught, that is 48 claims billed to the wrong payer, each costing a denial, a correction, a rebill and three to six weeks of delay. If a third of those visits needed an authorization under the new plan, sixteen of them may never be paid at all.
Against that, the prevention is a batch eligibility run that takes one person part of an afternoon, and a check-in question that takes ten seconds. This is the easiest cost-benefit case in the revenue cycle, and it is still skipped in many practices because January is busy and December is short.
The front desk work: re-verify everyone in January, not just new patients
The single most effective habit is to treat every Medicare patient scheduled in January as if they were new. That means running eligibility on every one of them in the days before the visit, not relying on the card on file or the coverage that was verified in October. Batch eligibility through your clearinghouse or PM system makes this cheap; run the full January schedule in the last week of December and again 48 hours before each visit. Anything that returns a different payer, plan or member ID than what is on file gets flagged for a call to the patient before they arrive. Because Advantage plans take a few days to load January enrollment, a run on December 28 will miss some changes that a run on January 4 catches, which is why you run it twice.
At check-in, ask the question directly: "Did you make any changes to your Medicare coverage for this year?" Most patients will say no even when they did, because they think of the Advantage plan as "still Medicare." Ask to see the card anyway. Advantage plan cards look nothing like the red, white and blue Medicare card, and a patient who hands you a new card from a plan you have never heard of has just saved you a denial. Scan the card, update the payer and member ID before the visit is checked in, and note the change in the account so the billing office knows why the payer differs from December.
The billing work: authorizations, referrals and the network question
Standing authorizations do not travel between plans. A patient with an infusion, physical therapy plan or imaging series authorized under one Advantage plan in November will need a new authorization under the January plan, and the new plan may have different criteria. Pull a list of every Medicare patient with an open authorization that extends past December 31 and re-verify each one in the first week of January. HMO-style Advantage plans also require a primary care referral for specialist visits; a specialty practice should expect referral denials in January from patients who moved from PPO to HMO products and did not know the difference.
Check your own participation status too. If the practice is not contracted with a particular Advantage plan, decide before January whether you will see those members out of network, refer them elsewhere, or start the contracting conversation. This is not a decision the front desk should be making at the window on January 5, and it is not one the patient should learn about from a bill in February.
| Patient moved from | To | What changes for the practice |
|---|---|---|
| Original Medicare | Medicare Advantage | New payer, network check, possible referral and authorization rules, plan copays |
| Medicare Advantage | Original Medicare | Bill Medicare and any new supplement; authorizations no longer needed for most services; Part B deductible applies |
| One Advantage plan | Another Advantage plan | New member ID, new authorizations, possibly a different network status |
| Advantage PPO | Advantage HMO (same carrier) | Same card design, new referral requirement, narrower network |
| Any | New Part D plan only | Prescriptions run through a new PBM; medical billing unchanged |
Get the panel data now while there is still time
If you participate with an Advantage plan, its provider portal or care management contact can usually tell you which of your attributed members have disenrolled or are scheduled to leave at year end. Ask for that list in the first week of December and again in the first week of January. It is also a good moment to look at care gap and quality reporting for the year, since attribution for the coming year is being set at the same time. Practices in value-based arrangements often find that the members who left were exactly the ones they had been working hardest to bring in for visits, and the members who joined arrive with a year of open gaps that count against the practice from day one.
Questions we hear
Can we bill the old plan if the patient didn't tell us?
No. Coverage follows the effective date of the new plan, which is January 1. The old plan will deny for no coverage on the date of service, and you will rebill the new one. If the visit happened out of network, the patient may owe more than they expected, which is an uncomfortable conversation that a January eligibility check avoids.
A patient wants to switch back after January 1. Can they?
If they are in an Advantage plan on January 1, yes, once, during the Medicare Advantage Open Enrollment Period that runs through March 31: they can move to another Advantage plan or back to Original Medicare with a Part D plan. Someone on Original Medicare on January 1 cannot use that window to join an Advantage plan. Point patients to Medicare's own resources or their State Health Insurance Assistance Program for the decision; the practice should not be advising on plan choice.
How much does this actually cost a practice?
It depends on the payer mix and the practice's follow-up speed. The direct cost is the rework: a denied claim that has to be corrected and rebilled, with the days it adds to accounts receivable. The indirect cost is the authorization that was never obtained under the new plan, which many Advantage plans will not approve retroactively. If you want help building an eligibility workflow that handles this every January, our medical billing team runs exactly this process for the practices we support, and our earlier piece on eligibility verification workflows covers the day-to-day version.
What to do this month
- Add the coverage question to the January check-in script now and train the front desk to scan every Medicare card again in January.
- Pull the list of every Medicare patient scheduled from January 4 onward and put the batch eligibility runs on the calendar for December 28 and January 4.
- List every open authorization for a Medicare patient that crosses January 1 and assign someone to re-verify each one in the first week of January.
- Confirm which Medicare Advantage plans you are contracted with for 2027 and write down how the practice will handle out-of-network members.
- Ask each Advantage plan you participate with for a disenrollment list in early December and early January.
