I saw an interesting talk by Alistair Cockburn (one of the Agile Manifesto guys), where he said he never once saw a succesful "agile transformation". Just too much resistance to change to have any chance to succeed.

What did succeed, is to create what he called a "new building", meaning a new subcompany (sometimes literally in a new building) with a different (agile) culture, and only letting in people that are interested in making it work. Slowly let the old company die, the new one grow. It takes years and years.