I don't get it...
Are you saying it's a C36 that's been swapped to a 2.8 engine and you want to swap back to a C36? Or are you saying you want to change it from a C36 to a C280? Maybe you're running the wrong VIN, or the VIN tool you have is wrong. I'm sure the dealer would mark it as a C36 if it were a C36. Used car dealers are required (by law) to give full disclosure of a vehicle so, if it has had an engine change to a non-stock (IE: Not what it was sold with) model, they have to tell you!