I think the reason Brian is giving almost totally new casts of characters is because characters used already tend to either have baggage to deal with from past experiences or have nothing left to give in terms of development. Now while the former of these can be utilised, such as in the case of Martin, it can also be quite limiting as it gives Brian less freedom as he must stay true to what he has said before while still using the character in the context of a working plot. The latter is even worse, however, as it means the character has nowhere to go on the most part, meaning that a character that was central in a previous book ends up on the back-burner, like Auma in "The Pearls of Lutra", unless he manages to provide some very original development as he did for Cregga in "(The) Taggerung". Carrying over is a difficult business when trying to keep the story good at the same time. It's all very good seeing our favorites again, but if it weakens the novel as a whole I'd rather have a fresh cast.
Of course, we don't know for sure that there won't be any younger creatures from "Loamhedge" over lapping into "Rakkety Tam" in their older seasons. Anyway, the cast we have at the moment looks good and also quite unified in the plot, something we've tended not to have for quite a while.