Is it legal to replace the old Audi a6 with a new one?

Legal. It is legal to replace the old Audi a6 with a new one, and it will be checked and registered by the vehicle management office, otherwise it will be illegal. If the car body color is changed, the engine is replaced, the car body or frame is replaced, or the whole vehicle is replaced due to quality problems, the owner of the motor vehicle shall apply to the vehicle management office for registration of change.