Thinking about how we did layouts with chalk before lasers... my blunder led to a backwards flange and a lot of teasing. | Boilermakers - Opinions Matter
Yeah, "just turn it over" is way too simple. It depends on the flange. If it's got a raised face or a flat gasket surface, flipping it makes the seal wrong. Bolt holes might line up, but then you're mating the wrong surfaces. You can really mess up a connection doing that.