you don't have to! the great thing about OS X is that it also installs Mac OS 9.1...
there are two ways to keep using your OS 9.1 applications in OS X;
1) In classic
If you try to startup an application that is not programmed for OS X, OS X automatically starts up the 'classic environment'; in fact, the classic environment is Mac OS 9.1 -inside- OS X, you will see a progress-bar that the classic environment is starting up, and after that (may take a while, just like OS 9.1 does) your 'classic' application starts up. You can distinguish classic applications from OS X applications by their window: classic applications have the grey look that macos 9 applications have.
2) Reboot in Mac OS 9.1
Although the classic environment enables you to run most appplications within OS X, applications that run within classic perform slightly slower than under Mac OS 9.1.
BUT! It's possible to switch from OS X to Mac OS 9.1 and vice versa. What you'll have to do in OS X is go to the system-preferences and choose 'startup disk'. Your computer will search for installed systems on your computer (including Mac OS 9.1) Pick Mac OS 9.1 from the list and restart your computer. Your computer will now start up in Mac OS 9.1
To switch back to OS X when you are in Mac OS 9.1: choose the Mac OS X disk in the startupdisk controlpanel and restart your computer.
Good luck! and have fun using OS X (be patient... more and more applications are being released for OS X, in a while many video-editing applications will be availlable as well)
