How to lower overall volume of a track that contains automation?

This happens to everyone eventually. It's even got a name: "fader creep". It happens as you're adjusting track levels; you want to hear a little more snare, so you bump it up a couple dB. Then you realize the bass needs to come up so you add a couple dBs there. Over time, multiple faders will gradually creep upward. Worse, you don't notice it's happened until late in the mixing process when it's a hassle to bring them all back down.
 
The safest method to deal with this is the Gain/Trim method. However, the wrinkle with that approach is it's going to mess with any dynamics processors you have on tracks.
 
Select every track (press CTL-A), pick any one track (preferably one whose current gain value is 0), and hold the CTL key down while you adjust that track's gain by however many decibels you need. All the other selected tracks will follow. Next, run down the tracks one by one, looking for compressors. For each compressor, you will need to lower its threshold by the same amount.
 
However, if none of your individual tracks are in the red, a much easier solution is the bus method. Route all your instruments to an Instruments bus, vocals to a Vocals bus and adjust their relative levels using the Gain sliders on those busses. But only do it this way if no individual tracks are hitting the red. Compensating for in-the-red tracks at the bus level isn't a good idea.