As things are now, is not possible to switch from Datastore to Firestore within the same project. Still, you may operate Firestore in Datastore mode. By creating a Cloud Firestore database in Datastore mode, you can access Cloud Firestore's improved storage layer while keeping Cloud Datastore system behavior. You may find more information of relevance by reading the "Automatic Upgrade to Cloud Firestore" documentation
page.
This discussion group is oriented more towards general opinions, trends, and issues of general nature touching App Engine and Datastore. For coding and programming architecture, as well as Java code samples, you may be better served in dedicated forums such as stackoverflow, where experienced programmers are within reach and ready to help.