Also, the changes were retroactive. For example:
Medical issued 4 years before the change to an airman who was 39 years old (or 29 for that matter, just before 40th birthday).
Airman let medical expire under the old 3 year rule (36 months was the old, not 40). Then when this new reg came into force, poof, the medical magically "unexpired" and was valid for another year.
That's because the medicals don't have an expiry date per se (unless you have an SI that is...otherwise, I defy you to find an expiry date on your medical). They have an exam date, and the regulations specify how long you can look back to find an exam date to be valid.