Responses to LynB and Mac, and thankyou both for taking the time to reply:
Firstly to LynB, I think you mistook the nature of my query: my problem was not with varying the initial velocity of a note but with changing it over its duration, as in crescendo or diminuendo. I can and do change note velocities, pitches, start times and durations when desired and I agree with you that the Score Roll Notation Window is a good place for these operations (but I call it Staff Roll mode of the Notation Window, unless there's yet another part of this program I haven't visited yet).
To Mac, no - I'm not dealing with audio in this situation, just MIDI. Does "CC" stand for "controller something" or just "controller"? I hadn't thought about using MIDI controllers on an audio track; I generally use gain change
(constant or variable) from "audio effects" in the tracks or audio edit windows, or sometimes "record mixer moves" in that window for audio stuff. I should say that I'm only just starting to encounter controllers and don't really know much about them yet, but I can see there's a lot to be had from a good working knowledge of them.
I've got the tools I need for my present purposes, but remain totally mystified about lines that I can draw in the bottom pane of the piano roll window but that disappear as soon as I release the mouse button (when view/edit is set to "velocity").What is their purpose and why won't they stay?