Before latest steps updating version
A code upgrade isn't required for platform updates.
For more details of the code migration steps, see Code migration.
A code upgrade is required when you deploy new environments as part of the process of upgrading your application to a new major or cumulative release.If your environment is already live in production, the source database is a copy of the production database.Otherwise, the source database is your most current database.Upgrading any additional Tier 2 or higher sandbox environments You don't have to upgrade any additional Tier 2 or higher sandbox environments.Instead, delete and redeploy them, and then make a database refresh request to copy a database from a Tier 2 or higher environment that has already been upgraded.
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If you're upgrading from Platform update 1 or Platform update 2, your developers must refactor the overlayering as extensions before you can upgrade.