Upset the apple cart
Definition 1 of 1
Pro Tip 1/3Unexpected Change
Use when someone's actions unexpectedly change the course of events or plans. 
Jane came in with a new proposal and totally upset the apple cart.
Pro Tip 2/3Negative Outcome
Often used to describe actions that result in negative outcomes or chaos. 
Trying to change the rules now would upset the apple cart.
Pro Tip 3/3Proceed Carefully
Suggests being cautious to not disturb a balanced situation or relationship. 
I don't want to upset the apple cart by asking for too many changes.