Hi Sri,
The question is fairly broad.
There are significant differences between the releases e.g: Administrative UI, policy store schema, Policy Server tools...etc, not to mention new features and fixes.
Hi ,
Also - R6 , R12, R12.5 is already End of Service.
Refer : CA Single Sign-On (formerly CA SiteMinder) Release and Support Lifecycle Dates
https://www.ca.com/us/services-support/ca-support/ca-support-online/product-content/status/support-life-cycle/indexes/ca-single-sign-on-release-and-support-lifecycle-dates.html
Regards,
Leo Joseph.