Comment by necovek

The fact that it took them 4 months to roll out does not mean this is the hard part: it's simply a coordinated rollout with incremental, staggered deployments and rate-limited migrations.

Changing data structures with that approach just takes its time as you avoid lock-step updates between components. Sure, by definition this type of development and deployment complexity is hard.

However, what I found the hardest is pushing engineers to adopt this evolutionary data structures mindset, and unless you do that right for the full team, someone will sneak in a backwards-incompatible change that blows the entire effort up.

So it is hard, but primarily for different-mindset-needed, and only then for technical complexity.