They would always contact your TA. That is a very rare occurrence but it can happen. Some people REALLY want their specific cabin have sailed in it before etc so they would never change you without asking. I have never known that to happen where a passenger was changed without knowledge.

Once I got a call and it was a good upgrade nice location. The client was "in the air" literally. I accepted on her behalf. When she got there she was NOT happy. She had an attachment to her particular cabin and it was a real mess on board but they got it back for her. I will never accept again without consulting the passenger I thought she would be pleased.