My idea about why Toa have been renamed masters: Toa is a Māori word. When LEGO got sued by the Māori people,LEGO might have gotten permission to keep the main character's names(Tahu,Gali,Lewa, etc) and their titles(Toa, etc) while the theme was running. When BIONICLE was discontinued, LEGO lost access to the words. When they tried to reclaim the ability to use them they were only allowed to use the names and the title Toa couldn't be used anymore. (Just my idea)