Hi Gordon,
I hope all is well my friend!
Not sure about this but maybe this setting/feature is where
things are getting tripped up;
In-service Upgrade Enhancement
The dual-banked firmware update feature allows the Cisco Unified CM administrator to upgrade phone firmware with a new load before resetting the new load to an Inactive load status. Instead of waiting for all the phones to download the firmware, Cisco Unified CM administrators can use the Switch Loads function to quickly switch from the old load to the new load in less time. Upgrading the dual-banked firmware reduces the bandwidth congestion and the delay in download during system maintenance while allowing Cisco Unified CM administrators to determine when to set the new firmware to Active load.
Note 
During dual-banked firmware upgrade, the previous Active load is swapped as an Inactive load. No change is made if the new load matches with the Active load settings. If there is no previous Active load, (fresh install), the Inactive load setting is empty.
A new area in Cisco Unified CM Administration Device Settings Configuration allows you to change the Active load and the Inactive load. Go to Cisco Unified CM Administration > Device > Device Settings > Device Defaults > Dual Bank Information area to change the load.
The In-service Upgrade Enhancement feature is supported on these IP phones running the SIP protocol:
•Cisco Unified IP Phone 8961
•Cisco Unified IP Phone 9951
•Cisco Unified IP Phone 9971
Cheers!
Rob
PS: Thanks again for all your additions for the CUCM comparison doc! I'll get back to
it one of these days
