Section 22 of the Principal Act is amended by replacing the proviso in subsection (1) with the following:
"Provided that where the Revenue Commissioners are satisfied that -
(a) the amount so estimated is excessive, they may amend the amount so estimated by reducing it, or
(b) the amount so estimated is insufficient, they may amend the amount so estimated by increasing it,
then, in either case, they shall serve notice on the person concerned of the revised amount estimated and such notice shall supercede any previous notice issued under this subsection.".