We are purchasing SQL Server 2016 and the respective CALs through a MPSA agreement and we are using a mix of device and user CALs. Since the average number of devices for each employee in our company is increasing, we only want to use user CALs in the future. What is going to happen to the already acquired device CALs?

In this case, Microsoft protects your investment by giving the possibility to transition between user and device CALs on a 1:1 level. However, the respective licenses must be covered by active Software Assurance (SA). Moreover, customers can only switch CALs when renewing the SA.

Source: Microsoft Product Terms, August 2017