We are planning to migrate from SAS V8 to V9. I have got references as what all changes will be required for this in the ETL server.
But we have a JAVA based application where we do have SAS Cube(OLAP) reports which uses SAS OLAP sever. I am not sure whether any changes will be required in the OLAP server or Java code for this migration.
The OLAP Server has totally new structure in SAS9. It would be strongly recommended to move forward to the new tools. But your old PROC MDDB would probably still work.
About your client, I have no clue. Is it a home grown java-app? Most likely you have some researching and testing to do. You might find some relevant information in the Migration section of support.sas.com: http://support.sas.com/rnd/migration/index.html