After upgrading my local test / dev installation (I hadn't touched it in over a year), I ran into some errors that were solved only by updating the dev files.

During the upgrade process, there is a screen that asks to disable IN_DEV mode. It may be worthwhile to add a note in the footer to remind users to upgrade the dev files as well once the upgrade is complete! (Or any other notification elsewhere, for that matter)

Or even better, the dev files get updated automagically