I don't think you can. As soon as faces have been analysed on an image, Classic will never analyse it again. Probably if you remove images and reimport them it might be willing to analyse them again but as far as I know there is nothing in the interface that allows you to control it for existing images.
What I found to have happened when I did this on family was that in the beginning it was seldome accurate with occasional success. But as I continued it got smarter and smarter and by the end it was mostly accurate. All I can say is that it was able to recognize my dad as an adult and as a kid. In the interup it would recognize a family friend with a beard and suggest it was my mom. Um, no, my mom never had a beard.
And I'm sorry, I can't tell you how many images I went through to get that level of results. Just keep with it and watch it learn.