Trigger was the way to go, works perfect!

Thanks for the input.

CREATE TRIGGER t_UpdateMemberStats BEFORE UPDATE ON members.member_stats
FOR EACH ROW BEGIN
IF NEW.news_comments < 0 THEN SET NEW.news_comments = 0; END IF;
IF NEW.photo_comments < 0 THEN SET NEW.photo_comments = 0; END IF;
END;

-Paul