Hi Nayab,
you can upgrade your dataontap version easly. each new firmware coming with new disk, shelf firmware. after upgrade completed successfully, disk and shelf firmware starting in background.
The important thing is you must read new ontap version description page. there are many alerts, pre requirement info at this page. You can find this page when you download new firmware from download.netapp.com.
Ontap 8.2 has a new improvements and new licensing model. may be you want to upgrade latest version of ontap 8.1.X . you can find details information on the new firmware description page or admin guide etc.