Delta loves to adjust its schedules, yes this is normal! It can almost always be used to your advantage since this allows you a free change to any reasonable itinerary up to 2 days before or after your scheduled travel date. When I'm booking a trip several months out, I just book the cheapest itinerary and then wait for the schedule change so that I can select the much more desirable routing/date/time. Even tho the desirable itinerary is often much more expensive, the change is free.

January is probably mostly stable now, but one last tweak is possible.