Photos will have detected most of the faces again, and already recognized many faces of most of the people as the same person. It is just not adding them to the People album all at once.
There is one big difference between the previous version of Photos and Photos 3.0: In the previous versions we could reject automatically detected faces (faces of strangers in the background, paintings on the wall, no face at all), but this is no longer possible in Photos 3.0. Photos 3.0 is keeping each and every automatically detected face around, and so we have to be selective and add only the faces of the persons we want to the People album. That is the reason why Photos is starting only with a few, frequently detected faces to initialize the People album.
The many hours I spent on 35,000 photos identifying faces I will have to do all over again? Tell me that is not the case!
Not all faces were named any longer, when I upgraded to High Sierra. And most of the manually added faces were missing.
My library is a bit larger, but it took me not long to confirm the faces again. For most of them existed already albums and I had just to step through the "confirm additional faces" routine. But I had taken the precaution to add keywords with the names of the persons to all photos in each people album, before I migrated the Photos Library to High Sierra. That made it very easy to search for one photo of each person to get the people album names again.