Since your 12 day trip will span a period from Christmas week (very busy) into early January (usually much less busy), I'd suggest that you book park pass reservations for all of the earlier, busier days, through New Year's Day. Then, if 1 or 2 of them get unseasonably warm, it's more likely to be an easy switch to move the park reservation to a January date. At least AK tends to be an available same-day option when there aren't peak crowds.
Also, take a look at your park pass reservations online. If you booked them fairly recently, they should all have links next to each one to "modify reservation" and not just links to "cancel". If they don't offer the option to "modify", consider cancelling them now, early enough to get back what you want. Rebook immediately so that you see the "modify" option, available since their IT upgrade. This will make switching dates a quicker, easier process later. HTH.