Implications of changing a user's license
Key Takeaway
Changing a user's license replaces their baseline access and removes their permission sets and permission set licenses.
You can change an existing user's license by editing the user. The change has side effects, because the new license controls which profiles you can choose and can take away access the old license allowed.
What a user license sets
Every user needs a user license to access Salesforce, and each user can have only one. The user license sets a baseline level of access: the features and objects the user can reach. Profiles and user permissions then adjust access within that baseline. Changing the license replaces the user's baseline rather than adding to it.