I'm a big supporter of "replacing one part at a time", and wish I had done that on a rebuild I'm just completing. In fact, I thought I was. We split our app in

I'm a big supporter of "replacing one part at a time", and wish I had done that on a rebuild I'm just completing. In fact, I thought I was. We split our app into 3 parts, rebuilt part 1, then part 2, but part 1 couldn't be released to customers until part 2 was done, and we kept our legacy system supporting the majority of our users until we are done with part 3, which is nearing completion now. I thought that was "replacing one piece at a time", but it isn't most users aren't touching it until part 3 is done, and at that point, they are experiencing a new system from scratch.