Yes, the Adobe DNG Converter will let you bulk convert Lossless DNGs to Lossy DNGs. In fact, I once wrote a script which scoured my entire library of DNGs to find all of my uncompressed DNGs and converted them to (lossless) compressed DNGs. Their DNG Converter is free and easy to run on a system which doesn't have Lightroom to bulk convert mass quantities of DNGs.
However, I usually do this within Lightroom, since it allows me to automatically add the converted files back into the catalog. All of my 2nd tier photos (not bad enough to be deleted, but which didn't make the final cut) get converted to a lossy DNG format. In fact, in most cases I convert them to reduced-resolution lossy DNG files (between 2mp and 6mp, depending on whether or not I think there is a chance of ever printing above 5x7"). I use one of my export presets to export the selected photographs to these reduced-resolution lossy DNG files (in a subdirectory of the original, named LossyDNGs) and then mark them as rejected. Once the export is complete, I delete all the rejected photographs.
This way, all of the photographs are still in my library, but most of them take up significantly less space, yet still have much of the flexibility of RAW files.