To be clear: If you have a DNG image in Lr, and you have made adjustments in Develop, by default those adjustments will not be reflected in the embedded preview images in the DNG. On the other hand, if you choose "Metadata -> Update DNG Preview & Metadata" (in Library) then Lr will embed new previews reflecting the adjustments you've made, as well as any metadata updates (e.g, embedding camera profiles, etc.).