On a bass track the highest velocity is 68, which is just wrong, lol.  I want to change all the notes so that the highest velocity is 127, but keep the relative velocities of the other notes intact.  Is there a simple way to do this?

Use the midi velocity scaling plugin. 

If that's the one found at Process > Scale Velocity, I'll play with it and see how it goes.

There is also the Midi Velocity plugin which is very flexible and will let you do what you want.
 
From the PRV, select your Midi track, right-click it and select Process Effect > Midi Effects > Cakewalk FX > Velocity
 
Once you've made your adjustments you can always ctrl + z to get back to where you were before if you don't like it and to tweak it some more.
 
The help file for this plugin is very useful

That's the ticket. A more powerful tool than the one previously mentioned. Thanks! (I did look at it before, but reading the help file helped me understand it much better  )