If you have a contract for a set or defined time (like a one-year contract), they cannot lower or change commissions until the contract has expired (unless the terms of the contract themselves allow the commission to be changed while the contract is still in effect). After the current contract expires, they can put forward a different commission, and it's your choice whether to keep working with them or not.If the agreement is not for a set time period, it can be changed on notice at will *going forward*--that is, from the moment you are notified of the change onward. But any sales made prior to notice of the change must be paid at the then-in-effect commission rate: retroactive changes for work done/sales made are not legal.