I have a similar use case but only need to update portion of the row. We basically perform single write (with old and new columns) with very low value of ttl for old columns. 

> From: jan.algermissen@nordsc.com
> Subject: Update-Replace
> Date: Fri, 30 Aug 2013 17:35:48 +0200
> To: user@cassandra.apache.org
>
> Hi,
>
> I have a use case, where I periodically need to apply updates to a wide row that should replace the whole row.
>
> The straight-forward insert/update only replace values that are present in the executed statement, keeping remaining data around.
>
> Is there a smooth way to do a replace with C* or do I have to handle this by the application (e.g. doing delete and then write or coming up with a more clever data model)?
>
> Jan