i think there maybe team changes also. its like a promotion of any kind your not going to stay with the same people if your promotion is to a different department. shikamara is a chuunin now so his missions will be on a class C and B in rank. i also think that once your at a chuunin rank teams are made up from the missions that they would be assigned. a good example would be when shikamara asked the forth that he wanted a team of only chuunin and juunins. so teams of genin reforming later would be likely since they worked together b4.