Why on Earth would you want to swap your RDBMS? If you're paying for Oracle it's because you need Oracle. And having paid for it you'd better use it.

'cause it's life. Sometimes changes happen due to cost, politics and various other reasons. It just happens.

As for what is swapped out, a well designed system can have any layer of responsibility removed and replaced due to light coupling. i.e. Dont' do database calls from your template system.