The seamless conversion is supposed to include an opt-out form. If you don't sign and send it back you are auto-enrolled in a plan of the CARRIERS choice.
Isn't that great? You will be enrolled in a plan of our choice unless you say otherwise. Perfect DC logic.
Of course the notice probably get's lost in all the other junk "Welcome to Medicare" mail that arrives on your doorstep.
But the real danger is agents even mentioning MA plan without proper consent a scope and 48 hour "cool off" period