Mercado Pago is a 12-year-old iOS app with 100M+ monthly active users in 8 countries. We changed its entire look twice: a brand swap from blue to yellow, and a full migration onto a new SwiftUI design system — rebuilt screen by screen, with a new architecture.

Each change needed the opposite rollout. The rebrand was big-bang: app-wide design tokens and theming let the whole app flip color at one scheduled moment, per country. The restyle was gradual: we rebuilt screens behind feature flags, one at a time, while the old ones kept running — highest-impact first, measured as we went.

The hardest part wasn't SwiftUI or tokens — it was getting many teams to adopt the new system and deciding when "done" is done enough. The payoff was concrete: dark mode had been impossible for years on a codebase full of hardcoded colors — once every screen drew from tokens, it became a switch we could finally flip.