Changing an SA cog is very easy... snap ring pliers are the ideal tool but a small thin screwdriver can also serve to remove the spring clip that holds the cog and spacers in place.
Exercise caution when removing the clip as it can fly away... covering the clip with your free hand during removal is prudent.
After you have done this a few hundred times it gets really easy and can be done in minutes... if you are unfamiliar with setting up the shifter you should review that too.
A massive change in cog size will also warrant a change in chain length and a smaller cog will probbaly require you to reduce your chain length as you may not have room in the dropouts to adjust the chain tension.
Sheldon Brown's site has all the information you would ever want on SA hubs.