Unfortunately one cannot upgrade from OSX Server 10.5.x straight to Lion Server.
There are two choices
- Upgrade to Snow Leopard Server (if you can find a copy & are prepared to shell out $400 or so. Then upgrade to Lion via the app store.
- Rebuild server as Snow Leopard workstation then upgrade to Lion server vai the app store
Option 1 is the easiest & less painful way to go. You can keep most of your server settings. But it costs $400 or so.
Option 2 is a bit more complex, but you save $400. Here is how I did it.
- Completely backup the target server – make sure you can access various configuration files
- Dig out a Snow Leopard install disk
- Follow these instructions to boot xServer from DVD. Put the Snow Leopard DVD in the optical drive, Use the first blue light option and replace the disk back in the drive when it ejects.
- Install Snow Leopard & do all the software updates
- Install Lion the App store, then reboot before the next step
- Install Lion Server upgrade from App store
- Restore server settings
Took about a day to complete.