Hello,
I'm not sure, but as far as I know - R=301 redirects - that's the way to go. I've done this once for a big customer. They had completely redesigned and changed their site structure for like 500 urls, all changed with 301 redirects. But because of lack of time, I haven't traced what impact it had on their rankings of new/old pages, after all it wasn't my job. But if you look at their web site now, it seems like they have good PR on their new urls:
http://www.fulltiltpoker.com
This makes me think, that Google applies PR to all new Urls as well.
I assume, Google takes home page PR=6, and redistributes all sub-level urls PR5 or 4 roughly. If you find answer to your problem yourself, please let us know, as many people here asking same question....
Hint: try changing 2-3% urls and see what happens.
Thanks,
Alex