Usurpation occurs when computer criminals invade a computer system and replace legitimate programs with their own unauthorized ones.

This statement is true. Usurpation occurs when computer criminals invade a computer system and replace legitimate programs with their own unauthorized programs in order to steal, spy and manipulate data.

Usurpation is unauthorized control of some part of a system.
So, if someone replace legitimate programs with unauthorized ones this means that the data in the system is changed from someone who is not authorized to do that.
In order to have integrity in the system improper or unauthorized change of data or resources should be prevented.