Hi Lars:
I contacted Customer Support regarding your question. Here's the guidance they provided:
There are some commands on the Ariba side they may have to run so the configuration files get updated correctly with the new version of Websphere. At a high level, the process will be to:
1. Upgrade WebSphere
2. Upgrade Ariba to SP27
3. Run the commands on the Ariba side per the readme of the service pack 27
4. Start the servers
If you need further assistance, you may want to submit an SR for additional customer support.
Regards,
Debbie Curtis-Magley
Ariba Exchange Community Manager