Metamorphosis Dan is mainly used for soul metempsychosis. When the spirit reaches a certain level, the quality can be improved (ordinary-rare-epic). When it reaches epic level, it needs reincarnation potion to improve its level.
You can use metempsychosis pills when you reach level 25, and the general spirit after metempsychosis will be upgraded to one level.
For example, Queen Guo has three qualities, which need to evolve to epic quality before she can use reincarnation pills again and become a higher-ranking general spirit. By analogy, metempsychosis needs metempsychosis pills to improve her quality and become a higher-level psychic.