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.