I've actually seen a FA make an elite move out of a bulkhead seat after the plane took off. I was 7A and he sat down on 7B. She politely asked for his boarding pass and told him to return to his seat. He was a Silver Elite. I was actually glad because 7C and I were looking forward to 7B being empty.

Although rare even if you are elite if you didn't change your seat prior to boarding they may boot you from ELR.