[kpil]

Pft, 2000 lines :-)

What you do is that you:

A) Realize that there might be years of hidden bugs, but also fixes to complex problems, speedups, kludges, weird changes to requirements, etc, hiding in the code, so even really creative unit-test might not cover real-world edge-cases.

B) Take as small bit as you can, and just refactor out the smelly code into smaller parts. Often a good name around smelly code helps a long way.

D) Iterate. Until good enough.But not more... Maybe you have more important things to do than rewriting ugly code that works.

There is a big risk that since you probably won't understand everything that goes on, and why - you will break something important.