Not sure if this is how it would work but by following the drain/drive/fill repeat method you describe, it would take you 9 quarts to do a fluid change and you would end up with about 6% of the original fluid left in the CVT (assuming 40% after the first drain, 40% of 40% the second and then 40% of that the third). If you were to change 3 quarts at each oil change you would always be leaving 40% used fluid in the transmission.

I have no idea what the intervals are for maintenance on the HCH but if you change your engine oil at say 10k miles, and your CVT fluid at 30k miles, you would use the same amount of fluid overall.

My own opinion would be to get as much of the old fluid out at one time than to leave 40% of the old fluid in there for a longer period of time. The old and the new fluid might not play well together.

Just my theory FWIW.