If you respecced to MM, you could still technically keep the exotic pets (say in the stable) but you could no longer use them as your active pet. You would need to go back to the BM tree and have the Beast Mastery talent in order to use an exotic pet. I haven't tried respeccing to MM while an exotic pet was out of the stable but simply dismissed. So, I'm not sure if you lose that pet or if it automatically goes to the stable or what.