POTUS kept Coats in as long as he did for a reason. He has either gathered the information needed for the new Acting Director to clean house, or he did something in the past bad enough to warrant a FISA.

My money is on him being a white hat. And with him already Senate Confirmed, POTUS can use him anywhere. And, as long as the DS thinks he is on 'their side' - or at least neutral, Coats is well suited to calm their fears as a member of a tribunal.