> Also the stamp has to be registered to have legal value, which makes it tough to change.
This is not actually true. Some stamps need to be registered (for example the stamp for corporation), but personal stamps for most applications don't need to be registered -- even for bank accounts. I have several and I'm always forgetting which one I used for my different bank accounts :-P.
One of the strange things about Japanese stamps is that if you let someone have your stamp, then it is considered that you have given them permission to do whatever they want with that stamp. The very fact that they have the stamp means that they are authorised. I got very angry at my previous employer (the government, no less) when my contract was over. They demanded that I give them my stamp I had used for stamping my time card. It happened to be the one I used for my bank account too (because I was clueless at the time!) It took me a couple of months to work around that. If you are ever working in Japan, treat your hanko (stamps) exactly the same way you would treat your encryption keys: use a different one for each application if possible.