In almost 30 years I've never seen anyone actually switch databases underneath an existing product. I have worked at one place where generic database APIs were required because it was a product that supportedf multiple databases, but no individual customer was really expected to switch from one database to another, that's just how the product was written.

I have heard of this happening, but it's the kind of thing that happens in one of two scenarios:

1. Very early in a product's lifetime the developer (probably a startup) realizes the database they chose was a poor choice. Since the product doesn't even exist yet, the switching cost is low, and generic database use wouldn't have helped.

2. A management shakeup in a very mature product causes the team to switch databases. This is, as you observed, usually part of a major rewrite of some kind, so lots of things are going to change at once. Also--critically--this only happens with companies that have more money than sense. Management doesn't mind if it takes a long time to switch.

   It won't go smoothly, at all, but nobody actually cares, so generic database use wouldn't have helped.