I personally use Apache Cassandra 1..6 version. but this version appears to possess some bugs that are fixed for 1..7. The main one fix involved obtaining the 'too many files' exception. I researched over it and discovered that 1..7 version includes a fix into it and 1..7 is much more stable than 1..6 version of cassandra. How do you perform upgrade without losing my GBs of information?? As well as please suggest whether 1..7 could be selected??

Cassandra upgrade instructions will always be situated in NEWS.txt in binary distribution. For version 1..7 it states:

Upgrading
- Nothing specific to 1.0.7, please report to instruction for 1.0.6

So, moving upgrade of the cluster ought to be all right to upgrade from 1..6