How do you change a 13-year-old React application serving tens of millions of monthly users when five developers are spread across three squads, there is no dedicated QA, no automated testing, and limited observability?
This technical case study explores how we navigate Smallpdf across multiple React paradigms, deprecated code paths, undocumented assumptions, infrastructure costs, and an enormous potential blast radius. We will examine how to build a useful mental model, introduce AI assistance without overwhelming it with context, assess risk, and migrate progressively without attempting a rewrite or disrupting the users and revenue depending on the system.
This talk has been presented at React Day Berlin 2026, check out the latest edition of this React Conference.























