First and foremost, do not assume that everyone who ever worked on the code before is a bumbling idiot. assume the opposite.

If it's code that has been running successfully in production for years, be humble.

Bugifxes, shortcuts, restraints - all are real life and prevent perfect code and documentation under pressure.

The team at Salesforce.com is doing a massive re-platforming right now with their switch to Lightning. Should provide a few good stories, switching over millions of paying users, not fucking up billions in revenue.