Typically, when you install an OS X revision, you have three major options:
- You can "update" which overwrites everything from your older system. In the Panther -> Tiger upgrade, there were a number of problematic new installs that were attributed to doing this, although it was never completely clear whether the issue was that old software was unstable in Tiger, that Tiger was buggy, or what.
- You can "archive and install," meaning you get a "previous system" folder to which all your old stuff is moved. If you do that, you can pick through it at your leisure and delete it when you have no more use for it.
- You can "erase and install," meaning you delete everything and start fresh.
The only curve-ball with Leopard is the fairly compelling rumor that the filesystem of choice could shift from HFS+ to ZFS. If that happens, you'll be encouraged to reformat your drive, and Leopard may or may not (probably would not) have a way to get your data onto the new partition without a backup. Most likely you would need to backup documents and so on, reformat, and then copy them into the user account in Leopard. Of course, ZFS as the default in Leopard is still a rumor, and it will almost certainly let you use HFS+ instead. The question is just what you'll be missing out on by not using ZFS.
