When you upgrade from an older Photos version to a newer Photos version the upgrade usually is using a new, improved face detection and face recognition. This requires a completely new faces scan.
Initially, the People album may be empty after an upgrade, but after a few days most of the faces should come back. Also, with a new, different face detection algorithm the position of some faces may be slightly shifted and the set of recognised faces may be different. Some faces may be missing, some additional faces may be recognised. For the slightly shifted faces and additionally recognised faces, Photos may be waiting for you to confirm the names, after the new faces scan has finished. This can take several days. For my library with 50000 photos it took two weeks.
- Open the album for each person in the People album and go to the bottom of the album. Click "Confirm additional faces".
- If Photos alerts you, that more names have been added than you confirmed, check the album for the person carefully, if photos has made a mistake and confused the faces of two people.
- If Photos is showing you wrong faces during the "Confirm" dialog, check the albums for the mismatches as well.
Photos 6 may not be able to use all previously assigned names. We are no longer allowed to assign the same name to two different faces in the same photo. So named faces may be missing, if you have a photo with two faces and Photos has used the name of a person for the wrong face. Sometimes some faces will be missing after an upgrade, that I have added manually.
When I migrated my libraries from iPhoto to Aperture I started to assign keywords with the name of the person in addition to naming the faces. Also I made screenshots of the people album to remember the original names I assigned. I continued this practice, when migrating to photos. Whenever I assign a name to a face, I am adding the name as the keyword. This is saving the assigned names across system upgrades and it helps a lot in Photos. It is not possible to see the full name of a person in the Info, unless we write the name to other metadata fields. The keywords help to check the faces after the upgrade with a smart album and to correct mistakes quickly:
For example - this smart album will help me to find all photos of a person, where Photos did not yet restore the person to its People album:
Checking with the opposite rules:
Keyword is not "Puppe: Tux" and Person is "Puppe: Tux" will tell you, if faces have become labelled as Tux, that are not Tux and need checking.
Here is more on using keywords to save the faces during an upgrade: How to use Keywords to Back Up the Named … - Apple Community
Using iCloud Photos can help a bit, to save the "People" album when upgrading, as the recent versions of Photos are syncing named faces with iCloud. But it can complicate things, if we have added faces manually on several devices and the circles of the manually added faces are in slightly different positions.
