I always use Find/Change but you have to know the highest and lowest value in the existing track. This way is not as convenient as those above, but it can be very precise.
Example: C3 currently goes from low of 85 to high of 100 and you want them all 10 lower. Do Find/Change. On the first screen click ALL and then select note values of C3 and C3 and velocities of 85 and 100. On the second screen click ALL and then select note values of C3 and C3 and velocities of 75 and 90.