When you purchase one WPBI Pro or Enterprise license you get 1 year of free updates and upgrades.
When the first year is over you can get additional updates and dedicated support with a small yearly fee.
Usually with the first purchase you also subscribe to the corresponding maintenance fees that will be billed yearly starting one year after the purchase. Of course you can cancel your subscription at any time.
Even after your subscription is expired you can renew it at any moment by simply upgrading your account from the free level to the one corresponding to your purchased license.
In case you want to upgrade from WPBI Pro to Enterprise you simply need to pay the difference between the two licenses and – after the first year has expired – the Enterprise maintenance subscription fees.