Answer:

A Basic To Extended Key is a code that may be used to change an existing copy of Foxhound Version 4 from the Basic Edition to the Extended Edition.

The "Basic to Extended" price applies to an existing installation of the Foxhound 3 Basic Edition that is being upgraded to the Extended Edition.

You can purchase a Basic To Extended Key here:

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