Is there any risk associated with using the 5-Star Special Enrollment Period?

Maybe.  When using the 5-star Special Enrollment Period (SEP), you must be aware of the type of Medicare plan you currently have (MA, MAPD, or PDP) and what type of 5-star Medicare plan you are choosing (MA, MAPD, or PDP) - because it is possible to lose your prescription drug coverage by joining a 5-star Medicare Advantage plan that does not include prescription drug coverage (MA) (see examples below).

As background...

• 5-Star Medicare Part D or Medicare Advantage plans are not available everywhere.
  If available in your state or area (ZIP code region), you can switch to a 5-star Medicare Advantage plan that does not include drugs (MA), a 5-star Medicare Advantage plan with prescription drug coverage (MAPD), or a 5-star Medicare Part D prescription drug plan (PDP) at any time during the year, provided you meet the plan’s enrollment requirements (for example, you live within the plan's service area).
  
  
• You can use this Special Enrollment Period to move from one 5-Star Medicare plan to another 5-Star plan.
  People already enrolled in a Medicare plan with a 5-star overall rating can use this Special Enrollment Period to switch to a different plan with a 5-star overall rating - perhaps for more economical or extensive drug and/or healthcare coverage.
  
  
• When does coverage of a newly-chosen 5-Star Medicare plan begin?
  The effective date of your new Medicare plan enrollment is the first day of the month following the month that the 5-star Medicare plan receives the enrollment request.
  
  
• Joining a 5-Star Cost Plan.
  You may also use this 5-star Special Enrollment Period to disenroll from a Medicare Advantage plan or prescription drug plan and enroll in an open Medicare Cost Plan that has a 5-star rating.

It’s Important That You Make a Careful 5-Star SEP Decision

CMS also created a coordinating Special Enrollment Period for prescription drug plans to allow people who enroll in certain types of 5-star Medicare Advantage plans that do not provide drug coverage (MA) to select a stand-alone Medicare prescription drug plan (PDP).  However, this coordinated enrollment is permitted under CMS rules on a limited basis, such as with a Medicare Advantage PFFS plan, but not a Medicare Advantage (HMO or PPO).

So, you may lose your prescription drug coverage if you move from a Medicare Advantage plan that has drug coverage (MAPD) to a Medicare Advantage plan (HMO or PPO) that does not provide drug coverage (MA).  If this occurs, you may need to wait until the next annual Open Enrollment Period (starting October 15) to get drug coverage, and then you may pay a Medicare Part D late enrollment penalty if you are without some form of creditable drug coverage for more than 63 days.

Also, if you move from a Medicare Advantage health-only (MA) or Medicare Advantage-Prescription Drug coordinated care plan to a 5-star rated stand-alone Medicare Part D prescription drug plan (PDP), you will lose your Medicare Advantage coverage and will revert to Original Medicare for Medicare Part A and Medicare Part B medical coverage.

Example 1
Acme Medicare Advantage plan has an overall rating of 4.5 stars in 2024 and a 5-star rating for 2025. You could use the 5-star special enrollment period to enroll in the Acme Plan starting on December 8, 2024 (after the December 7 close of the annual Open Enrollment Period (AEP), for an effective date of January 1, 2025.  But you could not use the 5-star special enrollment period to enroll in the 2024 Acme Plan for an effective date on or before December 1, 2024. The enrollment effective dates available during that period are before the calendar year (2025) for which the Acme Plan has received a 5-star overall rating.

Example 2
Acme Medicare Advantage plan has an overall rating of 5-stars for 2024 but loses the 5-star rating for 2025. You could use the 5-star special enrollment period to request enrollment in the 2024 Acme Plan for the first day of the next month until November 30, 2024, with the last possible effective date available being December 1, 2024. You could not use the 5-star special enrollment period to enroll in 2025 Acme Plan on or after December 1, 2024, as the enrollment effective dates available during that period are after the calendar year (2024) for which Acme Plan has received a 5-star overall rating.

Example 3
Mr. Miller enrolls in a 5-star Medicare Cost Plan without prescription drug coverage (MA). Under the updated 5-star special enrollment policy, Mr. Miller can enroll in the Cost Plan's optional Part D benefit (if the plan offers one).

Sources include:
CMS 5-Star Plan Rating Overview