The phrase "upset the applecart" is an idiomatic expression which means to cause a disruption or upset a well-established situation. However, there are many other synonyms that can express similar meanings. For instance, one could say "throw a wrench in the works," "rock the boat," or "spoil the party." All of these phrases suggest the idea of causing trouble or disruption to something that was previously running smoothly. Other synonyms like "shake things up," "alter the status quo," or "cause a disturbance" all convey similar meanings. Therefore, it is important to consider the context in which these expressions are used to ensure the right effect is created.