Spent most of yesterday, and part of the day before upgrading the three Macintoshes in our house. the two laptops went painlessly.
For some reason my PowerMac Pro had a HFS+ file system and Leopard requires a Journalled HFS+ file system. When I discovered that, I tried to quit out of the installer and reboot back into the old OS in order to save off my stuff. Unfortunately, the installer had damaged the disk and Disk Utility couldn’t recover it, so I ended up losing everything. It’s not a serious loss as everything of importance is recoverable, just a lot more work.
So lesson learned. We immediately ordered a one terrabyte external disk which I intend to set up with Time Machine, the backup software that comes as part of Leopard, to keep all our machines backed up to the new drive.