yes you need a separate software license key for upgrading the 6.1 to 7.2.
you can get the license key by writing to firstname.lastname@example.org and provide the serial number of the VCS boxes.
Also please note the software you download from cisco site when you do the upgrade. There are two version available encrypted and non-encrypted.
so downlaod the software which you have paid for
Also please follow the vcs upgrade procedure mentioned in the VCS clustering guide as when you take the vcs backup on x6.1 you need to run following commands before you do the backup.
the opiton keys would remain after the upgrade. however please note the same or take screen shot before you go for upgrade.