I'm not sure to what particular ambient type you make reference to. The most
"over-used" or classic type of change is the chiptune-like arp/thrills where
you modify really fast the pitch of the sound so that although you are on a
monophonic channel, it feels like a chord. What you can control depends on
the tracker's engine and there again it depends on what you are using..;
think of it like a CC implementation for synth. Tracker used to run on very
limited hardware.. running usually 4 to 8 monophonic tracks and limited
memory so all kind of tricks where used to make feel like there was a lot
more :)