Even if they change your final day to January, I'm still surprised - is it as per contract/Employment Ordinance/usual practice that employer "force" employee to use paid annual leave during garden leave?
I was under impression these are 2 different things.
My understanding was that whatever final date is, if annual leave was not taken by employee voluntary, it needs to be paid (aside from minimal necessary annual leave for some companies - usually 2 weeks).
Another thing - if you have first notice about final day in writing, not sure if it is up to them to change their mind and set new final date?