Mandrake provides a feature to upgrade the system instead of install it. Just put the first disk and choose upgrade
. However, I've never could successfully upgrade Mandrake 9.0 to 9.1 and 9.1 to 9.2. I always got problems doing that, either applications stopped working or ever extreme cases where the system turned into unbootable.
However, my bad experience should not scare you, but yes yes and yes, backup before doing anything.
One last hint, I would not really recommend you upgrading any previous Mandrake version to the latest one (10.0). They use pretty different kernels and most likely, a hole lot of applications will stop working (if the system will actually boot at all). I never did that, nor I am interested on 2.6.x kernel for the moment. I just didn't think they were stable enough yet when I used it with Slackware...
To sum up, I do, think you should upgrade if you like. I don't recommend you upgrading Mandrake 9.x to 10.x and I do, think backing up stuff is a must