Ask HN: Advice on a legacy database migration project?

I'm writing a blog post that aims to share real-world wisdom about what's involved in a legacy database migration. If we do this right, we'll curate advice that can help save someone else the pain you went through -- or guide them to repeat the things that worked out well. Maybe it'll come out as "7 things you should know before you start a legacy database migration." Or "tales of the lost." Let's find out. The key bit: What advice would you give to someone who's about to embark on a legacy database migration? ....and why is THAT the advice you offer? Now, tell me your own story: What was the project? What did you expect to happen... and what actually did? That is, what did you think would be hard... and was it? What did you learn from the experience? What were you proud of? What do you never want to experience again? DO include the technology just to make the details make sense; it's fine with me if this was a shift from Oracle to Postgres, though I may file off product names because this is about PROCESS and not brand affinity. Also give me the business context: You worked at a huge financial services firm that was still using a 20-year-old database where the source code was lost, or it was a ma-n-pa retail business whose data was stuck in dBase II and nobody knew it any more. Give me an idea of when it was, as well; 20 years ago is fine because the point is the lessons learned and THEY don't change. If nothing else, we can all relive our pain. That's healing, right?