If any vehicle which is registered pursuant to the provisions of NRS 706.801 to 706.861, inclusive, or has a declared gross weight in excess of 26,000 pounds, and which is exempt from the governmental services tax pursuant to NRS 371.100 ceases to be exempt after the beginning of the period of registration by reason of a change of ownership, the amount of the tax must be reduced one-twelfth for each month which has elapsed since the beginning of that period of registration.