My bad. I was looking at Choice Essential.

I just did a dummy booking on BWI-LAS to compare. I compared Choice Essential and Choice Plus.

At any rate, there are two types of Choice Plus - Refundable and Non-refundable. You change a Choice Plus nonrefundable without penalty, but you can't refund it. In my dummy booking, I saw these booking into N or S. Refundable booked into M.

Fully Flexible is the Y fare.

Choice Plus gets more perks than Essential and Choice, though most of these don't matter if you have status. Fully flexible gets more bags, MCE, and priority boarding, check-in and security.