I cannot see how the film was altered or why.
The only place you could claim it was is the cut at the start. Zapruder said he stopped filming and that makes sense, There was nothing during that time that would have been worth editing out, would there?
A somewhat larger GIF 9mb file at the end, but if you have the patience to wait you'll see the indiscreet paint job that was needed, in particular, beginning at the small white wall [on top] of the knoll should have been visible. Had Zapruder panned down Elm - following the limo - he naturally ... like all who've filmed down Elm from Z' perch follow the lowering gradient the limo was traveling down, which 100% brings the wall into shot. In 100% of every person who films the passage of a vehicle down Elm st they are forced to bring into shot the white wall. All except Zapruder!
But why? what was there to hide around this sequence of the 'movie'?
Maybe 'Black Dog Man' figure found in Willis 5 photo. (imo someone dressed in a Dallas Police uniform)
You can see Dark Complected Man on the curb of Elm in the very bottom left, and above him is your Black Dog Man (or cop in Dallas Police uniform).Whichever you want
So you simply paint it all out. Paint in some unrealistic size bushes, compared to reality. Enlarge the size of the limo, in sequence to the panning of the camera further down Elm. The purpose of the limo needing to be right up close, magnified in the foreground ... to easier paint out the truth about the little white wall and the man stood there.
5 days after the assassination the two FBI recreation films (black and white) with two filming zoom modes. It shows the obvious paint job done to the Zapruder film. There were NO corresponding high bushes in dealey plaza 22th November 63, as seen in the Zapruder film.
Frame 413 approximately below (depending if you believe Z-film isn't altered)
From frames 404 to 419 the Z-film becomes a joke. A painted mess. Hiding some considerable sins!