For the Upgrade the DB needs to meet the requirements of the new release e.g. DB version 10.2.0.4. I have had a training instance of PPM 7.5 running with this DB version without any issues. You should have a look into the sys requirements and compatibility matrix that correspondst to the 7.5 version you plan to continue to use, and make sure the doc corresponds to the service pack you have on your 7.5 instance.
Thank you for the reply. My db is already 10.2.0.4. I meant that there are 2 PPM schemas in that database. Both are using 7.5. 1 of them wants to upgrade to 8.0. Does the entire db have to be upgraded to 8.0 or can I just upgrade 1 schema?
I don't see a reason why you should worry then. The two instance have their own db schemes and if the access each other that would be via webservices right? The only disadvantage of such a mixed environment that I know of is that entity migration won't work between them, but that's no db issue.