ive looked on the resource guide and it doesnt say

drain and drop the pan. Plenty is left in the torque converter, etc.

When I did both my C4's I got more than 2.7L. Each of them took around 4 to refill. I bought 5 liters to be safe the first time and ended up opening 4 of the bottles.

draining out the old. You'd need to get all your new fluid into a big container with a hose that can draw in the new stuff. The old would come out the return line but you still need to change the filter by dropping the pan. If you fill and drain 2-3 times in a month you'll do basically the same thing.