aristotle - 2:18 pm on Mar 23, 2012 (gmt 0)
In my opinion this would be a risky move for two reasons:
1. I'm pretty sure that external outlinks to relevant content are a positive ranking factor that generally helps boost a page's Google rankings. But if you add nofollow tags to these links, then this rankings boost could be eliminated, causing a drop in rankings and traffic.
2. I think that Google may give less trust to websites that have a history of major changes. I also think that Google may give less trust to sites that have large numbers of redirects, deleted pages, no-indexed pages, and/or nofollow outlinks.