[michaelcampbell]

> “I am astonished they did not run the systems in parallel,” said a source at one of the major data vendors. “At least for a few weeks or months as necessary.”

I worked at a wall street trading firm for some years in the past. Our group was primarily working on replacing older mainframe applications. After the normal back and forth with development and QA, they would be run in parallel with the systems they were replacing for a minimum of 3 months. Development was not allowed to install, monitor, "fix" or otherwise touch them during that time. We either got a "go" or "no go" periodically, and we had to fix whatever the issues were or they never saw the light of day.