First, read this: Hazards And Safeguards for Software Rewrites. This is an infuriating article. The first part is nonsense. I wonder if it's intended to be read ironically? Maybe it's a list of bad ideas? Skip straight to the Safeguards for Rewrites section. It's really good.

Also, this Characterization Testing …

TL;DR -- To remove doubts and questions, rewrite it.

Many, many people are confronted with the request to maintain someone else's code.

Either it's open source, and you have to make formal PR's visible to the world.

Or it's "enterprise" in-house software, and you have to make PR's visible to …

Some years ago--never mind how long precisely--having little or no money in my purse... I had a great chance to do some Test-Driven Reverse Engineering on a rather complex C program. I extracted test cases. I worked with the users to gather test cases. And I rewrote their legacy app …

For the 757 Python Meetup group, someone proposed looking at some Python code they had which was slow. The code implemented a variation on the Elo chess rating system. It applied the ratings to other sports, and used the points scored as well as basic win/lose/tie to work …

Careful study of the HamCalc shows a number of quirks. Some are funny, some are just examples of the need for unit test frameworks. The Wikispaces for the modernization project is here: http://hamcalc.wikispaces.com/home

For example, the following line of code, in GW-Basic, will (usually) set Y …

Wow. The response to modernizing HamCalc was overwhelming. Apparently there are a fair number of people who also think that HamCalc is a treasure to be preserved, improved and added-to. If you're interested, start here: http://hamcalc.wikispaces.com/home You can ask to be a member of this wiki …

Step 1. Look at the original HamCalc. Even if you aren't interested in Ham radio, it's an epic, evolving achievement in a specialized kind of engineering support. It's a repository of mountains of mathematical models, some published by the ARRL, others scattered around the internet.

Step 2. Look closely at …

See this article: "The COBOL Brain Drain" in ComputerWorld. This article is very, very well written and repeats a number of common fallacies.

The fallacies lead to expensive, labor-intensive in-house software development and maintenance. Either there's a lot of effort poking at the COBOL code. Or there's a lot of …