A common source of confusion is the upgrade from 10 to 10d. 10d is in reality just version 10.1. The "d" was added (probably by the Symantec Marketing Department) to signify the backup to disk capabilities, which were already present in 10.0.

The upgrade from 10 to 10d/10.1 is therefore free, and you can use your existing license keys.

The installation files for 10.1 can be found at http://seer.entsupport.symantec.com/docs/279332.htm