All three of the Young Sannin replace their standard attack with the "100 chakra mystery" passive when you have 100 chakra before their action. It's not actually a mystery skill. It's similar to Kankuro/Sasori/Chiyo passives when they have a puppet. With a puppet, the standard attack becomes something different. The same applies here. None of the Young Sannin ever actually have their mystery skills change.