If, after the filing of a duplicate federal generation skipping tax return, the federal authorities increase or decrease the amount of the federal generation skipping transfer tax, an amended return shall be filed with the Department showing all changes made in the original return and the amount of increase or decrease in the federal generation skipping transfer tax. If, based upon such deficiency and the ground therefor, it appears that the amount of tax previously paid is less than the amount of tax owing, the difference together with interest, as computed under § 58.1-15, shall be paid upon notice and demand by the Department. In the event that the person required to file a return and pay such tax fails to file the return required by this section, any additional tax which is owing may be assessed, or a proceeding in court for such tax may be begun without assessment, at any time prior to the filing of such return or within thirty days after the delinquent filing of such return, notwithstanding any other provision of law.