Yes, you are correct, there may well be dependencies in your applications that rely on Sun’s implementation of ou, groups, responses attributes, schema, etc, that may not be identical in the new CA directory. Depending on our environment you may also have Sun specific or extended attributes and a directory structure that will need to be present in the new directory also. So the risk of just cutting over your applications from your Sun environment to a copy in the new CA environment is high.
I would suggest considering using a virtual directory as a layer of abstraction between the two directories. The right virtual directory will allow you to first capture the structure, schema, users, and groups of the existing Sun directory. By redirecting the applications to this layer of abstraction you can first proxy the exact backend Sun directory to ensure the applications still function. Once application functionality is confirmed you will be able to start to understand the structure and schema of the Sun directory and create in the abstraction layer a representation of that structure in a CA directory schema. Leveraging the abstraction layer you can extend the CA schema to meet attribute requirements, remap attribute labels, and create multiple new directory structures to support your current and future application requirements.
Throughout this process you identities still exist in the Sun Directory providing full application functionality. Then when you are ready you can re-path the applications to a new View in the abstraction layer to verify that full application functionality is supported. Should you need to fail back to the original View you simply re-path the application back to the View that represents the legacy Sun environment. Once all applications have been fully tested against the new Views based on the CA schema you can create the final structure in the new CA directory and then migrate the users and group objects.
At this point you have the ability to directly point the applications to the new CA directory or continue to leverage the abstraction layer to provide additional flexibility and potentially to provide legacy schema, structure, or attributes that are not supported in the new CA directory.
If this process is implemented effectively using a virtual directory capable of proxying, storing, remapping, restructuring, translating, and transforming your legacy Sun environment into your new CA environment you should be able to achieve this migration without reengineering your applications.
This video may provide you with additional information and things to consider. https://www.youtube.com/watch?v=CksKoqvOU0Q