(a) Except as provided in subdivision (b) of this section, no environmental impact statement shall be required for actions undertaken or approved prior to the effective date of article 8 of the Environmental Conservation Law.

(b) If, after the effective date of article 8 of the Environmental Conservation Law, the council proposes to modify an action undertaken or approved prior to such date, which modification may have a significant adverse effect on the environment, such modification shall be an action fully subject to this Part.