As I work in a large western store, and am one of the main Hatters in that store, I have cut down a great many hats. Depending on how old the hat is, and how much body (stiffness) is left in the hat, it may be more wise to use that one as a knock-around hat and purchase a new one and cut it down when you buy it.
Cutting the brim down is not all that hard, but having the tools to do it helps. I suppose you could rig you up a jig to do it somehow, you basically want to have something that slides around the existing edge of the brim with a fixed razor blade about 1/8" in. If you can do that, cutting it down is simple. I always start in the back (but never in the same place twice in a row if you must make several passes) so as to keep everything even. Put the guide against the edge, lower the blade through the felt, and go around in short movements until you're back where you started and repeat as necessary.
Once you've successfully cut down the brim, use a piece of 180 grit sandpaper and sand the edge to give it back the rounded edge it had to start with.
Hope this helps! I love being a hatter, I can shape, clean, and modify with the best in Amarillo. But having the tools to do the work makes things a whole lot easier.
