According to what I can find, The Joint Chief of Staff is appointed in October of odd numbered years and serves for 4 years. Therefore, I don't believe that the current one can be replaced until October. This also says that the position MAY be extended to eight years if the President chooses. It is possible President Trump set it up that way. ? https://www.law.cornell.edu/uscode/text/10/152#:~:text=There%20is%20a%20Chairman%20of%20the%20Joint%20Chiefs,beginning%20on%20October%201%20of%20an%20odd-numbered%20year.