Basically one of the parties has provided a written record of their version of events, if you don't agree with it, you need to speak up.
In the end they've told you the contract they want you to work under, they have provided you with a copy and you seem to be happy to carry on working and taking their money. If you have a problem, why didn't you speak up? So it's implicit that you accept the new contract.
Perhaps it's not the law in your country, but it's definitely not insane if you're in a grey area and in this instance the law sides with the written contract, not the avoidance strategy.