I just finished doing this. My developer and I would work on the the theme in an unpublished state which gives you a pretty good idea of how your assets will display. at inconspicuous times, I would publish, look, and then switch back. As we worked along, I would make benchmark copies of the theme to roll back to if needed. And I did. Once it was 95% there and at a slow time, I published the new theme for good and cleaned up the last 5%. The biggest challenge for me has been how images are handled between the Dawn and Debut themes. I checked before starting and learned that many of the Apps are not yet compatible with 2.0, so that can be a hidden stumbling block. I wrote each asking and then took the appropriate actions in preparation. One still caused a little heartburn, but they helped me get the last of their code out of the new theme after I published for good.