NLP algorithm: sentiment with specific guidelines
This seems like a straight forward classification problem: "Increased", "Decreased" (I assume "Neither Increased nor Decreased" can be a third category as well). Something simple like a Support Vector Machine (SVM) could work.

That being said, only 400-500 example cases aren't a lot so you might run into overfitting especially if you try to run whatever model you land on later down the road.