They'd change them for each track. Be trying to pull maximum power on the front straight in top.
GP classes would be changing each individual gear to suit the track as well. Running out of revs on the exit of this corner, alter 2nd a bit, not enough revs in the next one, alter 3rd a bit etc. Easy with a cassette style box.
I don't know how close to proddy bikes AMA has to be, but maybe they're allowed to run different internal gearing too?