Lesson drawn from Pope Gregory's handling of a 1600-year-old customer base. Ship simple and clear patch instructions on top of the existing rule rather than replacing it (the divisible-by-4 rule stayed; 100 and 400 were added). Take the opportunity to fix longstanding problems (dropping the 10 drift days). Minimise disruption by choosing your switchover day carefully (Thursday-to-Friday). Expect slow adoption — design the transition so old and new rules agree for a generation or more.