Are there any down-editioning rights available for Microsoft SQL Server 2016?

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Yes, Microsoft entitles license holders to perform down-editioning for SQL Server 2016. Please note that, independent of the actual installed/used edition of the software, the use rights for the acquired license/edition apply. The following rights are possible.

Source: Microsoft SQL Server Licensing Guide, as is 2016-08-28

Microsoft SQL Server Licensing Guide, as is 2016-08-28Download