How I rebuilt the Codable migration pattern across 4 iOS apps in 2 hours

I was adding a single new feature to DaysUntil: yearly-recurring events. Twenty lines of product code. But underneath, the data layer needed a pattern fix that turned out to apply to four apps, not one. Here's how I systematized it, and why this pattern compounds across every future app I ship. The trigger I added this one line to the Event struct in DaysUntil: var isRecurring : Bool = false Innocent, right? It has a default value. Existing JSON from older app versions don't have this key, so th