Why not? It's their land. I am not one of this chauvinists who claims native lands of other people. If Kaszubs and Silesians want to go free, I would have no problem with this. They are their own people, not Poles.
Unless they want to stay, also fine, since Poles, Silesians and Kaszubs are three subdivisions of one type of West Slavs (Lechs or Lechitians). Poland could be renamed to Lechia, for example, also.