Just my two cents. Yes, it would be quite a job, not as simple as changing STRING to WSTRING everywhere.
And it would be tough with my philosophy of major changes which it to do them in a series of small changes which keep things basically working. Even though this sometimes means re-visiting code areas repeatedly as we 'peel the onion'. Total rip-it-apart-and-put-it-together methods leave me cold.
As well, I'm not sure at all about the keyboard hook and how UNICODE alters that. Lots and lots to think about.