One of the major selling points of Drupal 9 is that the upgrade is really easy: fix your deprecated code, update your contrib modules, run some composer magic, and you’re done. While that usually is the case, some sites have their own set of hacks, oddities, and technical debt that make the upgrade process a lot more arduous. If you’ve found yourself in the middle of one such upgrade then you’ve come to the right place. This blog post isn’t a complete guide on how to upgrade to D9; it’s supplemental to the existing documentation found here.