While viewing the person album for a person with mismatched faces inside remove the wrong photos by selecting all wrong photos by holding down the command key wile clicking the photos. Now ctrl-click the selection and use "xxx is not in this photo".
Close the album for person xxx, quit photos to let it process the rejected photos, then open Photos again and open the album for the correct person. Try the command "Confirm additional photos" at the bottom of the album.
I'd hate to have to go picture by picture and retype the name. It's a lot of pics.
It should suffice to type the first few letters into the name field. Then the right name should appear in the list of suggestions.