Crescendo is the largest upgrade to ever happen on the Flow Blockchain and introduces many exciting new features such as EVM as well as the stable version of the Cadence programming language. All contracts written in Cadence must upgrade to Cadence 1.0 in order to continue working post upgrade. The upgrade process involves updating code to Cadence 1.0 and staging it to Mainnet, staged code will replace deployed code at the time of the upgrade. To learn more about migrating over to Cadence 1.0 check the Migration Hub.