When is it necessary to replace one class with another in software?

Replacing one class with another in software is used when:

• To change behavior without modifying the interface (for example, for improvements or bug fixes).
• To implement the "Strategy" or "Decorator" pattern for flexible algorithm replacement.
• To substitute the implementation with a more optimal or adapted one for new requirements.
• To ensure testability by substituting mocks or stubs for real classes.

It is important that the new class conforms to the interface or base class so that the replacement is transparent to the rest of the code.