I'm pretty sure a contract that has been modified without the knowledge of all parties who signed it is known as "some valueless paper with words on".
As long as you have your original, signed contract; and you employers have the aforemantioned paper with no signature, your contract is the binding agreement. Your employer can change your contract with your cooperation. If they want to do this, you are in a position to negotiate. For example, if they want you to change your work pattern, you can agree to this if they give you something like "more money" in return, and have a modification clause written (or a whole new contract).
There are caveats to what I just wrote - for example if they ask you to indefinitely change your work pattern because they need you to, and you agree, then it can become a verbal change. You'd have to have said something along the lines of "I'll do it for a month and then go back to my contracted pattern" in order to protect yourself. It's an implied contract or implied change of terms otherwise.
Note that if you do agree to a rewritten contract, you should compare it with the old one carefully. before signing, and make sure they haven't sneaked in any changes in other sections. You should also seek professional, or at least targeted, advice. A forum full of local footy supporters is not going to yield the most valuable advice.
I'd suggest you read
here and post any further questions
here.