That and it's way easier to go from retooling from sour to sweet than reverse, way easier to go from heavy to light than reverse, etc. Not suggesting it's just a flip a switch kind of change, but it's usually a net reduction in complexity in refining for both of those changes.

That goes against every thing I've ever read or heard. I'm no oil man, nor play one on TV, but I only know what information I've come across in reading or hearing in radio/tv. Maybe my googlefu is lacking, but a quick search still suggests this is the answer.