That's interesting Alan,
If you have your two ZDs in SR mode, defined each as peer to the other, did you also enable a common Management Interface?
Performing your firmware upgrade thru the Management Interface browser session, should upgrade both ZDs in turn, and gracefully
migrate and upgrade your APs between them.
On your Configure/Access Points page, AP Policies, have you entered both ZD real IPs under Limited ZD Discovery?
This also helps with the graceful move when ZDs pairs are upgraded (or should you have inter-network issues).
This combined with the SR peers from Configure/System, are the recommended SR settings.