Can a person be automatically enrolled into a Medicare Advantage plan instead of just Medicare Parts A and B?

Typically, no. Individuals who are eligible for Medicare (and who may already be enrolled in Medicare Parts A and B) are not automatically enrolled into a Medicare Advantage plan. If the client prefers Medicare Advantage, they must elect to enroll during the Medicare Advantage Open Enrollment Period. 

In some states, some individuals who are dually eligible for both Medicare and Medicaid can be automatically enrolled into a specific type of Medicare Advantage plan called a Dual Eligible Special Needs Plan (D-SNP), but that is not the general policy for the majority of Medicare-only enrollees.