Ask HN: How to proceed if you have a mind blowing idea

I have an idea which I am implementing right now. It is taking advantage of a feature provided by a multi billion dollar company who hasn't realized that it can make huge sum of money by making it a paid feature or limiting its use. I have signed a contract with the company and the contract is currently has no regulations on what I am doing, but I am afraid that the company will change the contract as soon as it realizes that I am using the system. What kind of legal protection I should try to obtain? I cannot share the idea with anyone unless I have fully implemented it? I am an owner of a small startup and am afraid that I might get screwed over. I just want to make sure that whatever I have done before the company changes it policy remains mine as mentioned in the contract. Any suggestions?