What are the conditions that allow to reassign Microsoft’s licenses to a new device or user within the 90 days period?

You are entitled to reallocate your licenses within 90 days if one of the following conditions is met:

1. Permanent hardware failure or loss
2. Termination of the user’s employment or contract
3. Temporary reallocation of CAL, Management Licenses and user or device CALs to cover a user’s absence or the unavailability of a device that is out of service
4. Through License Mobility which applies to specific server products, as long as they are covered by Software Assurance

Source: Product Terms_Microsoft_June 2016