No, you can only go forwards to newer versions. It used to be possible to export an XML and all the DPX/DRX stills, then import those into an older version of Resolve and manually rebuild the session from scratch. But the last time I tried it with a 15 Beta and Resolve 14.3 it did not work.
I have not tried it recently, so I'm not sure if this problem was solved with 15.1 or not -- and my system is tied up the moment and I can't check it today. Will see if I can experiment tomorrow.