I was wondering about the benefit of retaining guys from the previous coach. On one hand, you can see how a familiar face would make transitions easier for the players, but it might also perpetuate the "old way" of doing things which the new coach may want changed. You could imagine how a coach that is already experienced with the players could offer valuable insight into their psyches and their strengths and weaknesses, but then again, it may be that as a new coach you don't want to be jaded by the previous coaches' biases, interpretations, etc. Giving the whole roster a fair chance to show what they can do, without any bias or assumptions might be better. Watching film may in fact be a better way to get unbiased views, but then, for backups who never played, that won't be enough.
If an existing coach is truly skilled and valuable on their own right, then it makes sense to keep them. But keeping them simply because they know the players and have been around could backfire in a way, by making it take longer to establish the new way of doing things.