[galvs]

As a system matures it is easy to see common code and abstractions. Don't blame the previous guy.

Don't be tempted to start a new shiny clean system. This almost never works (unless in demise cases) and you'll be playing catch up with features on the original system for the next 10 years.

As others have said. You have to write tests, this will cover your back and you'll understand the codebase back to front once you have - Testing/Interfaces/Simplify/Naming.

Remember, all the business logic is there. At least you don't have to do the hard part... this is a cleanup operation.