The face tagging feature I believe is NOT a metadata field, but a Catalog function. Hence you can only hope to keep the ones where face tagging was also accompanied by keyword tags. If it was in a Catalog you should write the metadata to the file. It will then go into the IPTC keywords field. Did you convert the catalog? If not, use the old software and search for people faces, then add a keyword tag to those faces, then write the metadata to all the files. Then and only then will you be able to search the IPTC/other metadata fields for what will now be a PSE "face" in a metadata format. From what the instruction say, you cannot partially reset the face recognition, you will loose all the faces. So the face/name backup once complete into the metadata AND written into the files, is a good practice which will even allow other software to know what people are in your photos (but NOT where the faces are in the photo, that again is a PSE Catalog only function). As far as I know Picasa is the only software that automatically wrote the people names into the IPTC fields, and I was delighted when PSE picked that data up. But not as "faces", as "imported people tags".
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