At first sight it should be obvious. Cassandra is a distributed storage and you’re able to upgrade each node independently. But also it’s a kind of tricky, because Cassandra has so many concepts and moving parts. Introducing such a major change, you’ll be probably excited about how not to break one.
Also, as with every DB upgrade, the most important outcome will be your app behaviour. Protocol versions support might be removed from the future versions. Storage might work another way application doesn’t expect. There might be a lot of pitfalls. So, to start getting the benefits of upgrade, we have to be 200% sure that the application works. And at least it won’t work worse with database.
The whole process is straightforward but there do seem to be a couple places where you can shoot yourself in the foot.