You obviously have Not seen the complete Doug Horne interview of NPIC/CIA Image Expert Dino Brugioni. During that interview Brugioni describes the differences in the Z Film he Examined on 11/23/63 vs the Current Z Film.
So, what are they?
I'm not sure that anyone responded to my question, so, to the best of my knowledge....
In the Janney interview, Janney gives Dino a copy of Zapruder frame 313 to look at. Brugioni says that he remembered the "scatter" extending up higher in the film, 3-4 feet high. Janney then tells Brugioni that 313 is the only frame showing a cloud of blood and CSF. Brugioni responds by saying that he remembered that cloud on the film for a few frames, not just one. There is no point in either interview that I can find where he says that he thought the film was altered, only that the one frame he was shown wasn't how he remembered it.
That being said, there are a few things to consider:
If you do the photogrammetry, the streak that erupts from JFK's head in 313 and moves upwards at about 1:30 extends about 40" above the top of the head. Of course, that fits with Brugioni's description of the "scatter's" vertical extent.
For that matter, if they made enlargements to show what was going on in the limousine, then they likely cropped a lot of the background out. That would have resulted in an image where the "scatter" took up a much larger piece of real estate in the picture.
Janney was wrong when he told Brugioni that the cloud of matter only appears in frame 313. Any decent copy of the film shows it expanding and dissipating for a few frames after the shot. The cloud gets fainter with each frame, but that would be expected as the material exploding from the impact continuously spreads out over a larger and larger volume of space.
The 900lb gorilla in the room: Dino was 90 years old in 2011, and he was being asked about something that he worked on one night 50 years before. How well do you really expect him to remember it?