What process involves the changing of a program's internal structure without changing the way the program works?

Code Refactoring

Explanation: Code Refactoring is

A systematic process of improving a code without changing its functionality. It can be used to change a "mess" into a clean code.

Refactoring is the process of restructuring an existing computer code, changing the factoring without changing its external behavior. It is intended to improve nonfunctional attributes of the software it is expected to achieve this by continuously improving the design of code, hence making it easier and easier to work with.

Therefore the process of "changing of a program's internal structure without changing the way the program works" Is Code Refactoring.