They can't change the length of the contract without your consent, and if they did, that would be unenforceable; you'd only be held to the contract you'd signed. That said, what you need to watch out for is the following:1) If your original contract was for a set number of deliveries, and/or stated that any change in frequency of deliveries would change the contract length, that would be enforceable--so in changing the frequencey of the delivery, you may have changed its duration. (By halving the frequency, you could have doubled the remaining duration, for example.) Check the original contract to see if this was the case.2) If the document you signed for the change in delivery extended the contract, you would held to it--when you sign an agreement, the law presumes that you read, understood, and agreed to it, and holds you to what you signed. Review what you signed to see what its effect was.