Did this once time. Dont have the code thou.

First thing we did was to have a properrty on each page with the old id.
Then we made a application outside episerver that used the webservices or was it direct on the api dont remember.

This function syncronised one folder. Then we made it rekursiv ofc

Think the trick is to make it as an application so you have a bit more control on status, error messages etc.