Generally the necessary changes depend on what changes were made to the underlying source systems since OBIA is made to work with vanilla source systems and hence all changes must be reflected in OBIA.

Also security integration is always a crucial poitn though that depends even more on what happenes in the source system and how security is managed in general at the client.

There's no magical solution or answer to your question - that's the actual work which needs to be done and questions which need to be answered in such a project.