It's not really accurate to say that once the case is open to put in a hard drive, that an optical drive is easy to replace. In my experience with iBooks, it adds about half an hour of labour to the process. The reason for this is that the top shield doesn't need to be fully removed, but only folded out of the way of the hard drive. With the G4 iBooks, one also has to remove the bottom shield to fully remove the optical drive, which is added pain.
That being said, at least you're not swapping logic boards. That one is particularly painful...