The FlexConnect AP Upgrade feature allows for a selective upgrade process within a FlexConnect group. By not setting any master APs, the WLC will automatically select one AP of each model with the lowest MAC address to receive the upgrade directly from the controller. This ensures that the upgrade is distributed efficiently across different AP models in the group without manual intervention.
References := (CCNP Enterprise Wireless Design ENWLSD 300-425 and Implementation ENWLSI 300-430 Official Cert Guide)