Hi Umesh,
I would recommend taking a look through the release notes for 8.x to make sure you understand any new features and open caveats prior to your upgrade. This will ensure you are well prepared for any changes:
http://www.cisco.com/en/US/products/ps6120/prod_release_notes_list.html
If you have some more specific questions about the upgrade, let us know and we'll be glad to help.
-Mike