Hadoop was upgraded from version 0.20.2 to 2.0 across 5 production clusters storing over 1 petabyte of data and processing billions of events daily. Key challenges included managing configurations across heterogeneous clusters, allowing data movement between clusters on different versions, ensuring application compatibility, addressing capacity limitations, and performing the upgrade with minimal downtime. Extensive testing, parallel component upgrades, and rolling restarts were used to successfully complete the upgrade while maintaining over 99% uptime and without any failures.