I am not a therapist but I would think that any one's diagnosis could change. The person may have had an "adjustment disorder" due to something such as a divorce. A year or so later they may experience a severe depression due to the fact that they are no longer married or they just don't like where they are in life. It is no longer an adjustment disorder, but depression. But again, that is just my opinion, I am not a therapist or a certified coder. I just don't think it is that unreasonable.