If you're going to do a large web site redesign or migration, you are probably already planning on conducting a pilot. One natural approach is basically to consider the first site/section you migrate as the pilot, but not take the time to respond to issues uncovered during the migration (which would reduce acceptance of the project).  Three items to consider in a pilot are: 1) move toward your compelling vision, 2) launch a live site, and 3) reserve time to respond to issues.