You're putting a lot of time and energy into throwing away image information that you've worked hard to build up. Put off buying that new lens, get extra space if you want to go LRCC, or only import new images (for example). Of course, you are perfectly free to do it if you want to with your own photos, but I would not generally recommend this approach.
Thank you for your reply. I think you are right in that I have indeed spent a lot of time and energy in those pictures.
A suggestion. If people should want to upload their existing catalog to Lightroom CC, they could first make a full copy of their lossless RAW files to a cheap USB-disk and store it indefinitely in a safe somewhere for future reference (if ever needed). With this local backup they can safely compress any existing RAW files.
I want to direct your attention to an article by Google Photo's John Nack (18 June 2014) where he talks about the reasons for compressing his RAW files to lossy DNG and a follow up article by Mike Pasini (24 June 2014).
Good, now you say "local backup". That is very important for anyone reading this - don't even consider going to compressed DNG if you aren't sure you have a rock solid backup for your originals.
But I still think you'd be better off saving some money on a lens or your food bill and buying the extra cloud storage!
The lossy compression is only supposed to be used for archiving of images you don't plan to do any developing work with. The image is no longer a raw Bayer image. It is RGB and 8-bit. You are essentially saving a jpeg image with a .dng extension and some metadata.
If you want to shrink DNG images further while keeping the raw aspect alive you may consider using a program like Slimraw (see here: slimRAW: CinemaDNG Lossless and Lossy Compressor for Windows and Mac ). It has a lossless compression mode which converts the image to 10 bits in a non-linear fashion and encodes it losslesslym and it also discards the internal preview images which saves more space. I use this for my photos to shrink my library to the maximum without sacrificing quality.