Question about Netbackup licensing and Restore of old backups

Hi,

I have no experience of Netbackup.... But I have a customer that have used Netbackup for several years to backup a couple of OpenVMS clients. If they stop using Netbackup and alters to another backup system - what happens if they need to restore a 2 year old backup taken with Netbackup? Are they allowed to start NBU for restore purposes without license? Or do they need to have the product licensed for the whole period they need to be able to restore the backups? Someone who knows? 

Regards,

Björn R