Leon-

I've tried this and here's what I find:

1) A positive "Change by" value will only increase the volume if there was a previous negative "Change by" value
2) Change by cannot raise the volume above the level set in the mixer
3) I can mute an instrument at the beginning of the song but Change by cannot be used to activate it
4) I can mute an instrument at the beginning of the song but then when I want it to play I have to use Return to Normal for at least one measure and at that point it will play with the volume level set in the mixer
5) Thus if I want to increase the volume above a non-zero level at a later point in a song I must set the volume in the mixer to at least the level that I want at that later point and set a negative Change by value at some earlier point in the song so that the volume earlier in the song is at the level that I want.

If I follow these principles it seems to work but I haven't exhaustively tested this yet to be certain.

Thanks very much for your help figuring this out!