The following is straight out of the v5.9.1.0 release notes. As Rob pointed out, in order for Express APs to become standard upgrade to v5.9.3 required and latest release is v5.93.1 (all APs require to be on same firmware version).
Virtual Controller (VC) Enhancements
Heterogeneous AP management:
Adds support on the Virtual Controller (VC) AP to manage heterogeneous AP models for the 802.11ac AP platforms. The following AP models can be deployed in a heterogeneous AP VC scenario:
AP 8533
AP 8432
AP 7632/AP 7662/AP 7612
AP 7622/AP7602
AP 7532/AP 7522/AP 7562
Following are the supported adoption hierarchy:
AP 7662/7632 can adopt AP 7662/7632/7612
AP 8533/8432 can adopt AP 8533/8432/7562/7532/7522/7662/7632/7622/7612/7602
AP7532/AP7522/AP7562 can be managed by AP8533/AP8432 but not by AP7632/AP7662
Heterogeneous AP:
a) When using Heterogeneous AP management ‘anyap’ profiles must be used to configure Access Points which are not the same model as the Virtual Controller. These profiles must exist on the Virtual Controller prior to the Virtual Controller adopting and managing other Access Point models. Adoption and management of other Access Point models will succeed without any defined auto provisioning rules if the ‘anyap’ profiles are named “default-”. For example, an AP 8533 Virtual Controller will successfully adopt and manage an AP 7602 Access Point without an AP 7602 auto provisioning rule if the AP 8533 has an ‘anyap’ profile named “default-ap7602”. Otherwise you will need to create suitable auto provisioning rules for the other Access Point models if you choose to use a different naming convention for your ‘anyap’ profiles.
b) When using Heterogeneous AP management be aware that APs operating as Virtual Controllers have much less persistent storage than actual WiNG controllers. Because of this it may not be possible for you to update all APs simultaneously when using multiple AP models. The Virtual Controller may not have enough free persistent storage to hold the firmware update images for multiple AP models. The amount of free persistent storage is unique to every Virtual Controller installation. If your Virtual Controller does not have enough free storage for multiple firmware update files you will need to download a single appropriate firmware update file to the Virtual Controller, update the AP models that correspond to that firmware update file, delete that firmware update file from the Virtual Controller, then repeat the download/update/delete process for the next AP model.
