If the Russian fed punishes them then K&S should skate for Japan. They would be the top Japanese pair for now as Takahashi and Tran are up and comers still.
I'm not sure that they would be allowed to switch so easily. There's a lot of red tap surrounding nationality-switching, especially if you've competed at higher levels. Once a federation has invested a lot of $$$ in training skaters, they don't want them switching to represent another country, especially if they still have medal potential at major competitions.