I was lucky to be part of the team to do the Moodle migration from version 1.9 to version 2.4.There are many paths to follow to do the migration. But out goal was to have 100% data migration as the users are not that IT savvy and to make migration and experience of new Moodle with less effort as far as user is concern. We installed Moodle 2.4 on a new server. We design the front page (landing page) using themes already existed. The most tedious job was to do back of each and every course from old Moodle and restore it in new Moodle. Make sure the resources links and activities are working. Most of the time restore works. But when we have resources link to outside of the course it show broken links. Drag and drop feature was cool as you may select multiple files and drop them. Navigation in 2.4 is much faster and easier as compare to 1.9.
Still working on it will update as we proceed. Please share you experience also with us so that we could get benefited.