Upgrade instructions will be published when Daryna becomes stable. It is already possible to upgrade to Gutsy and Daryna but it's quite tricky and you're likely to encounter problems you would have avoided by following the upgrade instructions.
If your Cassandra/Celena system is stable, keep it that way and wait for Daryna to be stable as well before upgrading to it.
PS: Whether you perform a fresh-install or not, it's also a good idea to test the new release on your hardware with the liveCD in case some of your hardware is not as well supported as before or in case of other noticeable regressions.
I hope I didn't scare people too much here