My last job was in Seattle, on the Mitsubishi regional jet, the MRJ. There was a big team whose only task was to try to contain suppliers trying to apply similarities so that their system could be installed on the aircraft without any change. Sometimes I joked, “Are you calling similarity against the Wright Flyer?” It's amazing.
The technical rules should lead. By that I mean, you need to qualify, you need to test, and if you've made a change for this new aircraft that is significant, that's where the CPR comes in—minor versus major changes, or non-significant versus significant changes. You just bite the bullet and go and test it. It's amazing, the endless discussions I have seen in my career. They might as well turn around and test it; it would have taken less time and fewer resources.