Code Overhaul Plan

Outline a plan to convert a messy codebase such that, each and every function, class, interface, etc adheres to the SOLID principles. For reference SOLID stands for:
- S: Single Responsibility Principle: A Function is supposed to do only one task.
- O: Open-Closed Principle: A particular class must only be open to being extended. It cannot be modified. 
- L: Liskov's Substitution Principle: Subclasses must be able to extend Superclass's capability, not narrow it down.
- I: Interface Segmented Principle: Interfaces should be such that the classes will not have to implement functions that they do not need.
- D: Dependency Inversion Principle: Classes should depend on interfaces rather than concrete classes.

One more very important aspect is code readability. So make sure that you set up naming conventions for functions, variables, and classes to ensure each and every part of the code conveys what it is doing.

The plan should also ensure that we have steps to determine and test every function flow possible.