Max,
Contractually they can only do this if you agree to the change. (ie. if you agree to work on your day off.) By acknowledging it in crew direct, you are deemed to have accepted the duty.
Best thing is do not acknowledge anything in crew direct that you are unsure about.....now you know why. They will often put through a change, just on the off change that you might inadvertantly acknowledge it. They will change flights to an earlier start time, change O days to a work pattern etc. You do not have to acknowledge any of these on a G day. (On an O day, you are not required to answer the phone either - but thay can notify you when you phone at the end of your last duty.)
Then they will back themselves up with a phone call. You don't have to take the call - after all it's your day off. So they will then leave you a message, something like: "Max, we have a change of duty for you. Please acknowledge as soon as possible!". They know that you are new, and sooner or later your curiosity will get the better of you, and you'll check crew direct. You may even think that you about to miss a legal duty, or that they can legally change it. This is what they rely on. Once you've checked then they've gotcha!
My advice is do not answer your phone on any day other than reserve, and only ever check crew direct if there are no new notifications. Then, even when they are leaving you multiple messages (and you're really really worried!!) once the duty has passed you will never hear another thing about it. If you do answer the phone, you can politely decline....but it's always harder to do in practice, and you can feel like you need to come up with an excuse. Been there.
Anyway, if you've already acknowledged then you'll have to work this one old chap.....but next time
