Depending on the cassandra version you can use the copy cql command. To use that you need a cql client like the one that is distributted with cassandra.
First you have to copy the columnfamily A1 to a CSV file using:
COPY K1.A1 (column1, column2,...) TO 'temp.csv';
And after that copy the file to the new column family
COPY K1.A2 (column1, column2,...) FROM 'temp.csv';
Obviously you have to change the name of the columns to your names.
More information:
http://www.datastax.com/documentation/cassandra/1.2/index.html#cassandra/cql_reference/copy_r.html