They do adjust the depreciation code of a car from time to time, but there's no set time of the year when they do it, it can happen almost at any time and varies from model to model. For example, by my reckoning, they've adjusted the depreciation code for an F10 190MSport Auto 6 times since 2016 and a 2011-13 520d SE Auto 8 times since 2015, some resulting in upward and some downward amounts of VRT payable. These are separate to the annual adjustments that happen in January to reflect the changing age of a car.