Mint is based on Ubuntu and Ubuntu is based on Debian - this does not mean that Mint is Debian
Sort of shortened the reasoning.
You will not get Mint (Cassandra or any version) by adding Mint repos to Ubuntu, nor will there ever be a way to upgrade Mint by means of the "dist-upgrade" in the update manager, just because they are different.
Read also Clems comments in the thread about the Cassandra beta where he recommends a fresh install to be sure to get everything absolutely right. It's easy if you have home on a separate partition.