It goes like this:

1. Go to the last frame.
2. Edit: Memorize All.
3. Go to the first frame.
4. Edit: Restore All.
5. Cut the timeline down to 1 frame.
6. Save as a new PZ3.
7. Now you can increase the timeline to the number
   of frames you need in this scene.

(This sounds sort of complicated, but it takes only about 15 seconds to do.)

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