Official comment
After assigning the PM, you use this application to easily change the owner: http://www.clarizen.com/appsmarketplace/item/Simple-Find-Replace-Resource-Manager.html
The action is called "Find & Replace Manager" but it will actually replace the Owner. Another simple way to do this for small projects is to add the PM as an Owner to one task, copy that cell, and then paste it to the rest of the owner cells (similar to what you would do in Excel, you can paste to multiple cells simultaneously).
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