You are right, the only way is switch the order of your primary key. The reason is that, currently, your keys are in this order:
hashname1 key1 day1: counter_value111
hashname1 key1 day2: counter_value112
hashname1 key2 day1: counter_value121
hashname1 key2 day2: counter_value122
hashname1 key3 day1: counter_value131
hashname1 key3 day2: counter_value132
so to retrieve a range of days, Cassandra would need to 'skip' for each key. This is inefficient and not supported. You need to have them in this order:
hashname1 day1 key1: counter_value111
hashname1 day1 key2: counter_value121
hashname1 day1 key3: counter_value131
hashname1 day2 key1: counter_value112
hashname1 day2 key2: counter_value122
hashname1 day2 key3: counter_value132
Unfortunately the only way to do this is to rewrite all your data, and there is no inbuilt way of doing this in Cassandra. You could write a script to do this by reading in the data from one CF, switching round the order, then write out to a new CF and switch. If this needs to be done live it's harder but still possible.