TIL: There is no overlap in databases, never has been.
What you see is just an interpretation on the FMS screen, about the ending date. The non-overlapping view is the upgraded format, less confusing and more truthful.
Think about it, a procedure cannot exist and not exist at the same time. Or live in two different versions.
Handling the AIRAC swap inflight is easy with paper charts or EFB, but painful on some FMS configurations (all Airbus). That is, if the new data have actually been pre-loaded.
Relevant ICAO is Annex 15: Aeronautical Information Services. But not required for understanding, I guess.