The downgrade to Neutral suggests that Piper Sandler believes Immunogen's stock may not have as much upside potential as previously thought, which could lead to a short-term negative impact on the stock price. However, the increase in the price target to $31 implies a belief in the company's value and potential for growth, which could counterbalance the negative sentiment from the downgrade. The mixed signals may result in a neutral short-term impact on the stock price as the market digests both pieces of information.