If you already have activeCollab Small Biz, you can upgrade to activeCollab Corporate at any time. The price of the upgrade covers the difference in license prices ($250).
To upgrade simply:
Upgrading from activeCollab Small Biz to activeCollab Corporate does not extend your Support and Upgrade Subscription. Your Support and Upgrade Subscription will still be valid for a year, starting from the day you purchased your original license. After this period expires you can choose whether you wish to extend it or not.