You may also use the "Clip Automation" Envelop from the Edit Filter Pull Down.
This is non-destructive. It works like any other CC Envelop. the thing to note is that when changing values this particular envelop either adds to or subtracts from the current MIDI Velocity Value.
By default the envelop is set at 0. (means that no MIDI Note Velocity value is modified)
A value of 10 will add a velocity of 10 to the notes.
A value to -10 will subtract a velocity of 10 from the notes.
hope this helps.
cheers!!!