Without seeing the document no one can say for certain, but the *overwhelming* likelihood is that a form stating your availability is NOT a contract.
In the absence of a contract, your employer can schedule you for any time they need to. They are not obligated to schedule you only when it is convenient for you.
In the absence of a contract, they can change the terms of your employment at random.
You do not have to sign a new form, but your employer may terminate your employment if you do not.
Feel free to show the form to a local attorney, but I wouldn't be holding my breath on it being a binding contract.