The paper presents a video-based exploratory study that seeks to understand how developers modify UML class diagram layouts for better readability and comprehension of the system. Two diagram layouts showing a model subset from a Java open-source system are presented to six participants experienced in reading UML class diagrams. They are tasked to change the layout to make it easier for them to read and comprehend. The video is reviewed for major modifications to the layouts. Behaviors observed are presented. The eventual goal is to use this information to construct heuristics for automated layout algorithms based on semantics and architectural importance