There are lots of mechanisms to fast-track changes. If Ford changes the headlight assembly on the next year Mustang, they don't go and crash the car again. Th

There are lots of mechanisms to fast-track changes. If Ford changes the headlight assembly on the next year Mustang, they don't go and crash the car again. They (or a supplier) fill out the certification form showing that their testing indicates the new headlight assembly abides by all the regulations imposed on vehicles, and submit it to the relevant state and federal bodies. You can generally tell what revisions are straight-forward to certify and which are not by noting common year-to-year changes in a model.