One of my cats was born with a diaphragmatic hernia and we didn't know until she was like five years old. Organs were all over the place but things were fine. Stomach was in the upper chest, and the lungs had grown into the lower abdomen.

She ended up showing symptoms of not being able to keep food down (although eating slower seemed okay) and so we got it fixed for a pretty penny. It really seems like depending on where the organs end up, there might not be any issue. I was really impressed with how mechanical the problem is and how reliably successful surgery is. Presumably there could be complications if there's a real tangle.