A pension fund’s fixed-income portfolio manager is evaluating the relationship between credit ratings and credit default swap (CDS) spreads. While examining academic resources, they reference a study conducted by Hull, Predescu, and White, which explores the impact of credit rating changes on CDS spreads. According to the conclusions of this research, which specific type of credit rating change is found to exert the most substantial influence on credit default swap spreads at the moment it is announced?