The timetable should not change much on a fundamental stretch like HB - Flughafen, both in terms of operating times and frequencies. I would worry only for peripheric lines served by only one S service every hour...
This is a preliminary summary of the foreseen changes:
Normal December 25 schedule. December 25 2019 is public holiday (allg. Feiertag) and a Wednesday. Trains marked with (B), (C),†, and (3) will run on this day as well as all those runs without a special marking which run 365 days a year. For black box numbers see corresponding explanation on train schedule. Trains which only run on Sundays (7) will not run on December 25 2019.
Unfortunately there will be a new train schedule valid from December 15 2019. It will be available on the SBB Website on Oktober 4 2019.
The current situation is nevertheless a good hint when the first and last train will arrive. In between there a numerous connection from Zurich to Zurich Airport, also on December 25th.
From SBB "On 15 December 2019 there is a timetable change. In the SBB Online timetable, connections within Switzerland will only be visible from mid-October 2019.
But you can look at the 14.12.2019, because there will be no big changes."
Just book the flight. For the tiny, tiny chance that for whatever reason you'd not have a connection (as compared to today's sunday schedule) anymore which would enable you to reach the flight, you could always book a cheap airport hotel like the Ibis for that night before (around 80 CHF)