Hey Kevin,
Try not to mess around with changing sao. You could end up damaging your shamisen, and if you stretch or mess up the “square hole” in your shami’s dou, its a nearly impossible, very expensive repair that must be done in Japan.
Not all nagauta itomaki will fit nagauta shamisen. When each shamisen is made, the itomaki are fitted exactly to that shamisen. Changing these without taking it to a shamisen store to have them fitted will result in your strings slipping. Im sure the ones you have now work just fine. Buying random itomaki is like buying a pair of shoes without looking at the size. ^.^
Itomaki make no difference in the sound. AT ALL. I have ivory itomaki which had to be shaved and sawed to fit my shami at a shamisen makers shop in Japan. After all of that work that had to be done on them, all they are good for is making my shamisen look pretty… Sort of. -.-’’ Kind of frustrating, actually.
Last year, Kyle did some work on a Jiuta shamisen my school owns. It had synthetic skins, but the front one was coming off. Kyle replaced it with calf skin, and it actually sounds perfect for Jiuta, but is not the same as dog or cat for Nagauta. If you have a working shamisen, just stick with that one for now.
Sometimes used shamisen aren’t good unless they’ve been looked at by a shamisen maker. There might be a lot more wrong with it than a broken skin. In fact, that should be the least of your concerns when buying a used shamisen, but thats another story.
Happy practicing!
Joe