Not so. There was a recent case of exactly that. The Captain concerned was told he had to resign from CX and then rejoin as a DEFO on COS'99 on the base.
Correct Messiah.
To clarify my post, I was referring too a simple case of downgrading from Captain to FO position keeping ones seniority, COS & entitlements
exactly the same as currently on.
To this request the company will say NO.. cannot.