The downgrade to Underperform suggests that Evercore ISI Group believes Aflac's stock may not perform as well as its peers in the near term. This negative outlook could lead to a decrease in investor confidence and a potential drop in the stock price. However, the increase in the price target to $78 provides a mixed signal, indicating that while the stock may underperform, there is still a potential for some price appreciation from its current level. The net effect is likely negative in the short term due to the downgrade.