What are the requirements for SQL Server license mobility?

In order to be eligible for license mobility – and this applies to all Microsoft products for which license mobility exists – an active software assurance is required. The actual license mobility can then be handled either through Windows Azure or an Authorized Mobility Partner. It should be noted that the licenses concerned may only be used by and for the benefit of the licensed customer.

Source: Whitepaper_Microsoft_License Mobility Through Microsoft Software Assurance 2016-07-05.pdf