Last minute Medicare Open Enrollment questions

[intrepidity]

1) is this true: if disabled and receiving SSDI prior to age 65 and you did NOT sign up for PartC (Supplement, not an Advantage plan), when you turn 65 do you get another chance to enroll (that avoids the underwriting health checks etc)?

2) if you have Original Medicare but during open enrollment switch to an Advantage plan, can you revert to Original next year if you find you don't like the Advantage plan? That is, is there any downside to switching to an Advantage plan, long term,?