If you have a whistle with a removeable fipple pulg, you can have an adjustable lower chamfer. native american flutes have one of the first adjustible windways, and there are some others also. You may have to make a whistle to have one with a removeable fipple plug. Why would a person want one. It saves makeing many plugs to see which chamfer works to your liking. There might be a tune you would like to play that favors the upper or lower octave. It can change from toot whistle down to sax or obo.
If your plug can slide in and out put some non harding clay where the chamer is and make the edge square. Play it than change the edge to a beval. This can be done with a stright paper clip or a X acto knive. If you have a difficult time trimming a very small amount. Wet the paper clip or flat side of the X acto knife and just press a beval. The wet keeps it from sticking. You will also find out the effect of the plug going in to far or not far enough. If it will not slide in any more you can add clay to to it.
You may need to put bees wax or chap stick on the plug to reseal it each time it is removed.