The Change Potion is given by Seda, on the 8th floor of Divine Beast Cave. Seda gives it to Toan after their duel is finished. There is a cat on the floor, Which Toan protected, the cat runs off and then turns into an Atla. The atla contains a Stray cat.
The Stray cat can be placed in My house and when you have all the other required parts for My house, the event will trigger and your Mom tells you that she found a cat. The cat can be found in your bedroom upstairs.
Press square then select Changing Potion, The cat will transform into Xiao.
- The changing potion is probably an ancient article with which Seda seems eager to part. This is the point where Seda's course of action is vaguely being entrusted to Toan and the Fairy King.