Some updates are done in place, some are not. To give a specific example - integer updates are always done in place. The most common reason that a document has to be rewritten is when it is updated size is too big to fit into its previous space. Then it has to be moved.
Could be, but that article just says that the update is "in place." Which may mean that just the changed int is written, or it may be the entire document.
http://blog.mongodb.org/post/248614779/fast-updates-with-mon...
I have found "source diving" a scary experience with Mongodb in the past.