Yes...you can download an install DNG Converter 6.7 (the most recent version) and reprocess older DNGs into newer DNGs and use the newer options.
However, there are some things you should know. Lossy compression will reduce the file size but it requires doing a de-mosaic of the original raw file and produces a linear DNG. The compression (at this point, it may change) is JPEG about level 10. So, a lossy DNG will be smaller for sure, but...it's not without consequences...the upside it lossy DNG still behaves like a raw file that is much smaller. You still have the ability to do raw adjustments and since the lossy DNG is in a linear gamma, you do still have the ability to do substantial highlight recovery...
Also note you can actually downsample the lossy DNGs...but you need to understand it does literally downsample the original image resolution and you end up with a lower rez image. You really REALLY don't want to do that to your only "original" raw file...
I do understand the side effects - the reason I was looking into this option was for minimizing diskspace of the many keepers-but-not-5star shots. I'm shooting lots of Northern Lights pictures in wintertime and have tons of images which are not really 5stars but also not throwaways...and my budget's not big enough for those nice 6-disk RAID enclosures you have sitting under your desk
I also figured out a way to do this in LR - a bit counterintuitive but it works...Library-Convert Photo to DNG (eventhough it already is a DNG...) and setting the flag on Delete original after successful conversion.