What are the requirements for a step-up from a lower to a higher level edition?

You are entitled to a step up if the following prerequisites are fulfilled:

1. A lower level edition license must be available
2. Active Software Assurance (SA) must be available
3. You have to be enrolled in one of the following volume licensing programs: Open License Value, Select License, Select Plus, Enterprise Agreement, School Enrollment, or Enrollment for Education Solutions (CASA+EES or OVS-ES).

Source: Volume Licensing Brief_Microsoft_Step up Nov 2013