I'm surprised nothing about functional tests were explicitly mentioned in the article. There were some hints about "engineering the heck out of your migration scripts" (paraphrased) which could be taken as setting up functional tests.

If there is a good tests infrastructure re-writes can be done with a heck of a lot of confidence. Plus there is the benefit of breaking the re-write project up along test lines which helps in not creating the pressure for a, "huge one-time switchover".