What I really want to do is to physically change the key. So that's mean I must void my warranty to make this change, am I right?
It's unclear whether that would void the warranty. Apple's warranty prohibits "unauthorized modifications", but whether they would consider moving key caps around to fall in that category isn't certain. My guess, and this is just a guess, is that unless you were to damage the keyboard while changing the keys around, it's perhaps unlikely that it would void the warranty. But I can't speak for Apple, so again it's not certain. If you want to do this modification, you need to understand that should problems arise with the system, you may run into difficult with warranty support.
why can't the staff help me to change this? It is really a minor stuff.
Because Apple doesn't support that sort of modification, so their employees cannot help perform such an unauthorized change. Apple provides and supports the keyboard as it's shipped only; if you change the keys around, you do so at your own risk and you're on your own. You might be able to find an independent dealer or service center who could help you, but that would be strictly up to them; again, Apple won't support the change.
