You have a perpetual (unlimited) plesk licence right? It came with 100 WPB seats included. We have (had) several of these also and started reaching 100 seats + usage. After discussing with with our parallels rep they tell me there is no way to upgrade WPB on a perpetual key. you will have to contact parallels and esquire about exchanging your license for a leases one in which upgrades from 100 to 1000 or higher are simple. Good luck.