The 3-month window of OEP, Jan 1st until March 31st, is a time for people that may think they are stuck in the Advantage Plan that they selected during AEP in the fall to make changes. If you have tried out your Advantage Plan and do not like how it is working out for you then you can switch to a different Advantage Plan or drop it and just have original Medicare.
There are several reasons why people change Advantage plans. It could be because your doctor changed networks, or your medications are not being included in the current plan's coverage.
There are restrictions in OEP that you should be aware of. You can only make one switch. This means that once you move to a different Advantage Plan or drop it for basic Medicare, the change is generally locked in for 2021 (unless you meet an exclusion that qualifies you for a special enrollment period). Additionally, this three-month window does not allow you to switch from one stand alone Part D prescription drug plan to another.
Meanwhile, dropping an Advantage Plan in favor of basic Medicare often means losing drug coverage which means you would have to enroll in a stand-alone Part D plan. This is important because if you go 63 days without the coverage, you could face a lifelong late-enrollment penalty that gets tacked on to your monthly premiums.