Sounds like you've been listening to too many red riders. The YFZ is fine you don't have to change anything that is unless you want to (jetting, exhaust, etc.) What do you mean chain problem? As far as I know there hasn't been any problem.
I think that he means the chain roller relocater. The roller will get worn down or off if you jump alot or just compress the rear end while riding. But this is not a must. Also, jetting. Some say that the bike is lean from the factory, and that you should rejet. Go to www.eskimo.com/~daddog/index.htm