I think this is legit, it says it's from medicare.gov. Some think that if you choose Medicare Advantage, you're stuck with it for life. I was pretty sure I was offered a choice of Traditional Medicare but am happy with my Humana plan so didn't follow up. According to this you can switch from Medicare Advantage to traditional medicare if you choose to. The only thing that seems a little odd is receiving this now instead of during or just before open enrollment.

If you or someone in your household has a Medicare Advantage Plan, you can make a coverage change during the Medicare Advantage Open Enrollment Period, which runs from January 1–March 31 each year.

You can:

Switch to a different Medicare Advantage Plan; or
Switch to Original Medicare (and join a separate Medicare drug plan).

Explore Your Options

You can switch only once during the Medicare Advantage Open Enrollment Period. Any changes you make will be effective the first of the month after you request to join the new plan.

There's no need to take any action if you're happy with the plan you have. But, if you want to find a plan that better fits your needs and budget this year, now's your chance to shop for a new plan.

Sincerely,

The Medicare Team