I don't have insider info, but 787s are in short supply, which I assume is what is driving the use of a 772. Pragmatically, whether they operate a 789 or 772 wouldn't seem to be a huge deal for the passenger experience.
I also just got a "flight change" notice from my HNL-DFW flight in August. Oddly, they just changed the flight number from AA 8 to AA 115. Times and equipment remained the same, at least for now. I wonder what the rationale for changing the flight number but nothing else is?