To get back on topic, I don't think Squarebear is right.
If an employer decides to change a contract, it has to give you (the employee) notice. It is in essence saying the old contract is finishes on 30th September and a new one starts on 1st October, and I guess that it told you this some weeks ago?
It also appears to be saying that you cannot continue working under the old contract after 30th September (has it said that in writing to you?). Therefore the old contract finishes at that date and you can walk away.
That's the line of argument I would take with the prats.