Tool "Scale velocities"
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mmg1
In "Edit" menu, add beneath "Remove overlapping notes", a command called "Scale velocities...".
A dialog box features:
Radio-button for range, 3 options: selection, track, song
Spin control: Minimum velocity (default : computed minimum velocity)
Spin control: Maximum velocity (default : computed maximum velocity)
When user validates, velocity in selected range are rescaled from specified minimum to maximum velocities.
"Scale..." should be then renamed to "Scale durations..." for sake of clarity.