What you describe probably won't happen. Most programs, if you set the fader to make a move to a specific value at a specific time, it will always go to that value, even if you change the starting point value.
But sometimes you want to lower or raise the overall level of a track, while still keeping the existing volume automation, so that the levels will change proportionally, (but at a softer or louder overall level.) This can be done fairly easily in a number of ways. One would be to put a plug-in on the track that has a volume or trim adjustment built into the plug. You could set the plug-in values to neutral (so that you aren't actually compressing or EQing, etc. the track) and just lower or raise the volume within the plug - which changes the overall level of the track without affecting your volume automation moves.
A second way would be to route the output of your track into the input of a new track, and use the fader on the new track to raise or lower the overall volume. Again, the automation on the original track would proceed unaffected.