It would be trivial to create your own tracking id and map it the existing UUID, so this is nothing major to worry about just more work for the developers.
It wouldn't be hard, but it would require developers to update their software to associate teh UDIDs with the new IDs, and then for the users to download the update, and them for them to run the app before Apple take away access to the UDID. A load of data is going to get lost there