This is very interesting, but I don't want to be the first to migrate a production 2.x DB to 3.0. Can someone else do this and write an article on how hard it was and if it actually ended up working? :)
To change storage engine to WiredTiger, you will need to manually export and upload the data using mongodump and mongorestore. But you can do it for members of a replica set separately. http://docs.mongodb.org/master/release-notes/3.0-upgrade/
