There are great answers with many takes on experience, but I would say, the correct answer is you can do the upgrade in a non-destructive way. I blogged a bit of this on the site blog:
I would recommend getting an external drive that's as fast an interface as you can (perhaps a FireWire 800 desktop drive) and installing the system there. You could then know that you can always shut down and go back to the OS on the internal drive if the external drive runs slower than you had hoped. An external drive will always be a "little" slower than the internal, so you might not get a full experience of the speed you would get by upgrading the internal drive, but this lets you spend a little time making sure your apps run well - no matter the speed and then you can make some "speed" measurements to decide if you are making a step up, a step down or just a sideways step, but newer options and more recent security fixes.