I've moved from sqlite to serialization via Lua on one, when clarifying the problem made it clear that using a relational database was actually a poor fit. Most of my examples aren't database-related, though. I agree that switching databases doesn't happen much, I'm just noting that having a major, conceptually distinct part of a project entangled with the others is usually a sign of other problems. (Maybe that's just my experience maintaining legacy codebases, though.)

I think that getting preoccupied with moving from one database to another for scalability reasons is usually wishful thinking -- it's like worrying about all the positive attention you're getting.