I was leaning towards the theory that Millard was planning on swapping the VINs, but I still have my doubts. In Ontario all vehicles need emission tests every two years. During that test the VIN number is entered in a computer linked to the government database. The fact that one truck was a diesel and the other wasn't would have immediately been detected the first time either vehicle needed that test. And Millard would still need to get a green ownership for his red truck which would now have Bosma's VIN, as the red trucks ownership would presumably gone with Tims repainted truck.
So even though that plan sounds pretty straightforward, it's not really as simple as it sounds. I still think the windshield removal tool was ordered to get at the vin plate as most people don't remove a windshield to paint a vehicle, I just don't think the plan was to swap it with Millards VIN.