Case of volumes I believe. Classical case of pricing theory.
At the moment car sales are tanking, so with low volumes Govt gets a low revenues via duties. New changes should rejuvenate the market leading to increased sales. This means more volumes. Should volumes exceed pre-duty change times then the Govt earns more. Its a case of dropping prices to stimulate demand. Basically Govt is reducing the "price" of our right to own a vehicle in SL.
If anyone has access to this data, volumes of cars imported into SL annually for about 5-6 years we could get a better idea how much Govt would benefit.
Secondly more vehicles on the road = petrol consumption goes up. With the tax on petrol this means more taxes for Govt. This assumes price of petrol stays same - unlikely as it is subsidized.
Second hand market might see a drop over the course of 2-3 months. Basically everyone is waiting for Govt to gazette this change.
In any case I for one am thankful I sold my Ford Laser a year back!