Answer:

A Extended Upgrade Key is a code that may be used to activate Foxhound Version 4 as an Extended Edition copy after upgrading an existing copy of Foxhound 1, 2 or 3.

The "Basic Upgrade" and "Extended Upgrade" prices apply to any previous version of any edition of Foxhound whatsoever (Rental, Basic, Extended, Free Trial, Beta Test, etc.). In these cases an existing Foxhound database must be available to be read by the Foxhound 3 installation process.

You can purchase an Extended Upgrade Key here:

See also...
What is a "Basic Upgrade Key"?
What are the differences among the different editions of Foxhound: Rental, Basic and Extended?
What are the differences among all the activation keys?