Enquiry on Product key for upgrade

Hi Support Team,

I am planning to upgrade our current CRS 2008 service pack 0 (SP0) to service pack 7 (SP7).

When try to install using the downloaded SP7 installer, it prompted to enter product key.

I cannot proceed further when I used the current SRS 2008 product key.

Please advise further how to proceed from here.

Thank You