As it is mentioned in the official documentation:
Warning: Once you select a database mode and execute the first write on the database, you cannot switch. You can't use both Native Mode and Datastore Mode in the same project.
In case you executed a write, you cannot switch anymore within the same project.
EDIT: In the situation you did not write anything this types of switches are available:
Datastore -> Firestore in Datastore Mode
Datastore -> Firestore in Native Mode
Firestore in Datastore Mode -> Firestore in Native Mode
In your case, if you really find consider that the change would worth it, you need to create a new project.