Do you know what will break and how to be ready for them?

Most of your reports (built in and custom) will be broken as the database schema has significantly changed.

To be ready for this, do a test migration and update any custom reports against the new database schema. That way, when your live server is up and running, your reports will be ready.

See John Socha-Leialoha's post for a good write up about what has changed and how to get your reports working.