Except for proposals for legislation, as provided in CEQ Regulation (40 CFR 1502.9(c)), the Commission shall publish supplements to either draft or final environmental statements if:

(a) The Commission makes substantial changes in the proposed action that are relevant to environmental concerns; or

(b) There are significant new circumstances or information relevant to environmental concerns and bearing on the proposed action and its impacts. In the course of a trade regulation rule proceeding, the supplement will be placed in the rulemaking record.