I had my FC seat changed without my knowledge nor consent during boarding.

When I was last to board on a flight, my seat was taken, 6B. The FA said there had been some issues with a passenger, and they needed to sit in 6B.
1A was his previously assigned seat.

I countered with the fact that I had a medical condition, and I want to fly with my medication handy, under the seat in front of me, on an aisle. And said in a quiet voice to the FA that I was not going to take a seat until this was resolved.

In the end, the person in 3D took the bulkhead, but the entire commotion with me was due to a similar circumstance.

It seems they are within their powers to do so. If they aren't, this is the place that will tell us so.

Oh, and I am an EXP.