I'm not totally sure yet. I think you can add fields to a table without issue, but if you change the 'primary key', aka the 'partition key' in CQL 3 terms, then you're changing how the table is organized, and I think there's some downtime to make the change.

One thing painfully missing right now is "IF NOT EXISTS..." which is available in the next major Cassandra version (2.0), but it's not stable yet.

Confusing: CQL is the SQL-like language Cassandra uses. CQL 3 is the new one, supported by Cassandra 1.2 and 2.0. Many clients only support CQL 2, but that's different from Cassandra (or C*) 2.0. Sigh