That is definitely strange. The last time this happened to me, I found out the change occured because one of my originally booked flights was cancelled and I was rebooked on a completely new flight. Given that your original flights are still showing up, I find that strange as well. Also, when this happened to me, it was August when they were trying to trim their schedule.

For an itinerary in December, I've been downguaged from a CR9 to a CRJ (sucks), but no "red button" on my itinerary, although I did receive an email announcement since the times were slightly altered by like 5-10 minutes.