Calibrating Crossmedium Adaptation

When is it justified to change the source material when adapting a piece of media to a different medium? In recent memory there has been sweeping changes to Game of Thrones, based on the George R. R. Martin book series A Song of Ice and Fire. Prior to that we've all seen changes that have altered entities like Harry Potter, Super Mario Bros., and Watchmen, so I'm sure we all have experience with the concept. In addition, during the news segment we discuss Nordic going full THQ, trouble in the land of Minecraft, The Legend of Zelda, and more!