The downgrade to Underperform suggests that Evercore ISI Group expects National Storage's stock performance to lag behind its peers or the broader market, which could lead to a negative short-term impact on the stock price. However, the increase in the price target indicates a belief that the stock still has potential to grow in value, albeit at a pace that may not keep up with market expectations. The mixed signals could result in some investor uncertainty, but the downgrade is likely to have a more immediate negative effect.