Hi,
BSP Software has a tool specifically designed to tackle this use case, even with mismatched CAMIDs, and will allow you to easily move users in Cognos to the new Authentication Provider, map the users when appropriate, and gives you options on how to handle users that exist in only one of the namespaces. You also get access to the full suite of BSP Cognos admin tools, so that you can clean up any security or content issues while you do the migration, so that when users log into the new provider, they're greeted with a more efficient and effective Cognos than they had before.
And we can do all of this with minimal manual work, and you can complete a namespace migration within hours (once you're ready to press go).
Check it out
http://www.bspsoftware.com/products/metamanager/smss/I'm happy to answer any questions or set you up with a demo of the tool and help get you started.