1. Everyone is happy with the situation as it is. If you try to change it, you’ll be upsetting the apple cart.
2. The new president was installed and immediately upset the apple cart by appointing his own people to various positions in the administration.

Similar to: rock the boat. Whereas rock the boat usually describes a situation in which the people involved do not want a change, upset the apple cart can be used to describe any situation. The expression dates back to the Roman Empire and was originally ‘upset the cart. ’ ‘Apple’ was added by the late 1700s.