Wait until you get much closer to your travel date in December. It is way too soon to do anything about tickets for December even if you have decided already that you will not fly. It is likely that one of your flights will be get cancelled or re-scheduled; then you call and ask LX to remove the remaining active flights in your reservation but keep the ticket/flight coupons (the ticket will remain valid option). Subsequently, in January when seats for your December 2021 travel dates have been released, you call again to rebook. That should not trigger a repricing (but there is no guarantee).

If you rebook your ticket now/without flights being cancelled or rescheduled, you will will most likely be asked to pay fare difference and you can't rebook for december 2021 yet.

If your flight is not cancelled or rescheduled, I fail to see how you will accomplish a rebooking to December 2021. Then I guess the only option is to ask for voucher.