can they change a job describtion mid stream with out notification

I work for a fire department in west tennessee. in 1996 I started the process for promotion to assistant fire marshal. when the process was over I was awarded the position. the job classification was to be the rank of battalion comander with the job title of Assistant Fire Marshal. Every thing was fine until dec 10, 2001. a comp study was done which I was droped out of the position of battalion comander and placed in my own class far less than that of a battalion. when I greived the process I was told that my classification was change in 1998 to a captain rank. with this changing, my pay is now far less than that of what it should be if I was still a battalion. My question is can they change a job description mid stream with out notification to the employee. also I assumend the position under one pretence and now am working under another without notification until now.