Any luck with this? I am looking for a similar process which looks to update DNG previews only for those not current or if not full-sized. My hope is that other applications (book making, emailing, family members viewing, etc.) can use the fully current preview and avoid me having to maintain a "mirror" set of JPEG images in a sister directory. In any case, the other option is to turn DNG previews off (saving file size) and just use Rob Cole's TreeSync3 plug-in to keep the mirror JPEGS in sync.
Personally I would Try:
1. Try open DNG again in DNG Converter and try make a copy with a different settings, for example large JPG preview.
2. Try find Setting "Update embedded JPEG Previews" in Camera RAW preferences