"The secret of change is to focus all of your energy, not on fighting the old, but on building the new"
A big bang approach to any kind of technology migration is not advisable. A series of deliberations have to take place before the eventual and complete change over. Migration from RDBMS to Cassandra is no different. Any new technology replacing an old one must coexist harmoniously with the old technology, at least for a short period of time. This gives a lot of confidence in the new technology to the stakeholders.
Many technology pundits provide you with various approaches for the RDBMS to NoSQL migration strategies. Many such guidelines are specific to particular NoSQL data stores, paying attention ...