Herb Caudill’s Lessons from 6 software rewrite stories is an extensive analysis of the rewrite projects of companies like Basecamp and Trello, why they succeeded or failed, and what we can learn from them:

Once you’ve learned enough that there’s a certain distance between the current version of your product and the best version of that product you can imagine, then the right approach is not to replace your software with a new version, but to build something new next to it — without throwing away what you have.

Eventually all product managers face a decision like this, so I found these case studies invaluable.