Could using an orifice for the wrong gas type damage a water heater?
Nat gas orifice is significantly larger than propane orifice , so the heaters have been exposed to more heat /flame than design ; that could be a problem. I learned this the hard way;  hooked up a nat gas grill to a propane bottle. On the lowest burner setting flame came out around the space between the top and bottom of the grill. In round numbers the gas molecules take up the same volume ( Ideal gas law) . In the nat gas molecule you have one C and 4 H to burn , in a propane molecule you have three C and eight H to burn. So the nat gas orifice is larger to let more molecules through , it lets the same larger number of propane molecules through producing more heat/flame.