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What is the best example of a large legacy codebases that has been significantly improved?

I am interesting in finding out what is the best example of a large legacy project that has gone through a "successful" quality improvement process. By success I mean a step change in quality.

Before:

  • Large: greater than 1MLOC
  • Legacy: no automated tests
  • Poor quality: high complexity, high coupling, high escaped defects

I am looking for specific details on what approaches actually helped in the real world. The reason I find this interesting is that there is a lot of information about tools and techniques for improving large legacy codebases, but I haven't come across any real world case studies. Most advice is on the micro level, and while helpful, don't convince many people because of a lack of evidence it can help at the macro level.

I think that this information would be very motivating for developers and managers in this situation.