There is a migration process from 8.1 Server Edition to 9.0 virtual Server Edition. You have to buy a new virtual Server Edition license and swap the existing licenses to the new system ID.
It is important to install virtual primary server first via OVA file and set IP addresses, host name and time zone. The virtual server SID depends on these settings and will change if you change one of these values.
Then you can get the SID of the virtual server and can start the swap process.
When you get the licenses you can start to migrate you existing systems.
You can start the initial setup for the expansion systems again but I'm not sure which settings will be kept and which settings will be deleted. You also can change the needed settings (license server, SCN trunk, directory etc) manually.