I did, almost twice. First time it was a home-grown layer on top of the DBMS (MS SQL Server at the time). After an investment we've got a requirement to switch

I did, almost twice. First time it was a home-grown layer on top of the DBMS (MS SQL Server at the time). After an investment we've got a requirement to switch to Oracle. We anticipated that (or something like it) when designing the system, so it went smoothly. Second time it was using MySQL from Sun workstations, with the idea of eventually switching to Oracle. That's why "almost" - we knew about the requirement in advance, so we didn't create hard dependencies on MySQL. Yet we didn't constantly test our system vs. Oracle either. Admittedly the latest of these happened more than 10 years ago.