As it's a maple, and will leak sap if you cut it in spring (when you would chop other trees), do it in late fall (Nov/Dec.).

DO NOT CHOP in late fall. You will get significant dieback from the chop down the trunk, especially if you have a hard winter. Chop in very early spring before active bud extension. Seal the wound. Protect from hard freezes. The tree will lose sap, but it isn't going to kill it. It's a myth that trees "bleed" to death from sap loss. It might look alarming, but it's not as bad as it looks.

Chop it when it is dormant or prior to bud break if you want it to bud back. Sure you risk die back or sap loss.
Except that and cut it back higher than you normally would to begin with. Remove the unwanted portion the following summer after the first new growth has hardened off.

mpro, where are you in Kansas? I am in Overland Park, soon to move to Ottawa if all goes well. First, remember that japanese maple is less vigorous than trident maple. Trunk chopping is still okay with a healthy specimen, but make certain you have lots of roots. Do the chop in early spring before bud break, chop it higher than where you want the cut to be eventually, and seal the cut. Don't really do any root work at the same time if you can help it. If you chop it and leave it in the ground, you should get strong growth.

Where tridents are strong growers, japanese maples are a little more delicate. Tridents are a warmer-zone tree, japanese maples are a colder-zone, so tridents suffer from freezes, japanese suffer from heat. If you are going to keep japanese maple bonsai in Kansas, be prepared to have a shade house to keep the tree from burning up. Also wrap the pot in white towels or t-shirts in the 100 degree weather to keep the roots cool. Check out my trident trunk chop.

Here is a link that might be useful: Trunk chop

I think your Trunk Chop link is broken bonsaikc .

On a different note, Since you just purchase the Japanese Maple, how about plant it and let it grow and establish one season and then do the Next season to avoid two stress at one time - Trunk Chop and repot :)

(I am a newbie, so that above statement is just a logical guess. I have no practical experience other than the fact I killed two Bloodgood trying that :-(