AMO - https://addons.mozilla.org/ was originally written as a PHP web application, ported to Python / Django 1.1 in 2010, more or less maintained over time and only recently got much more traction because of Firefox Quantum and Mozilla's move to WebExtensions.
The talk will show our approach to maintaining very old code, handling refactoring, adding new features as well as feature/code removal while slowly upgrading our way to a Python 3 and Django 2.0 ecosystem and why we chose that approach over a rewrite.