The upgrade fee is 50% of the standard price.
Company X purchased 50 Tardis 2000 (v1.3) registrations in August 2001. They were entitled to a free upgrade to v1.4 but now wish to upgrade to v1.5.
The upgrade fee is
50% of 50 x US$4.00 = US$100
If, however, the company also wished to run the software on additional PCs they would be required to pay the standard fee for those registrations.