Maybe. There are certain events that would make you eligible to change plans outside of the Annual Enrollment Period. For example, if you retire after 65 and no longer have coverage from your employer, move to a new service, or are turning 65, you would be able to make changes to your plan.
These life events can happen anytime during the year, so insurance providers can make exceptions. However, if you know you want to change coverage, AEP is most likely the best time to do so.