Yes. In Lightroom, the way to batch-compress existing DNGs is to select the images and use Library > Convert Photos to DNG. In that dialog, choose to convert RAW files, delete originals after successful conversion if you want to replace them, select lossy or lossless compression as needed, and do not embed the original RAW file.

Lightroom can also create compressed DNGs automatically when you import using Convert to DNG.

Important: Metadata > Update DNG Previews & Metadata is not the command for compressing files. That updates the preview and metadata embedded in the DNG, but it does not reduce the file size by compressing the image data.

One limitation noted in the community answers is that Lightroom may only let you do this one folder at a time.