Yea, with the Transmission it's not to critical if you have it a little over full. The front diff is more critical. It doesn't like to much fluid, or to little. Just ride it and see how it feels. If it starts acting strange in any way, you might need to remove or add some fluid.
Ideally, you want to get the ATV warmed up extremely well to change to fluid... get the fluid warm so that it flows and drains well.
Get it on a level surface and let it drain for a long time. Push up and down and side to side on the front of the ATV to try and get as much out as possible.
Put the new stuff in SLOWLY. The gears are very close to the drain hole, and if you put it in to fast, it will come back out on you and you think you have it full.