The first scan only happens, when you import a photo for the first time. Then Photos will analyze each photo and search for faces. This face detection is only done once, after importing photos. Or when a Photos software update modifies the algorithm for the face detection. Then Photos will scan the photos again.
But you can reset the face recognition, the scan for identical faces and the databases with the added names to the detected faces. That is, what will happen, when you delete all albums from the People album. Phots again look for identical faces and discard all names you previously assigned.