Rename the Airport Utility 6 first, to "AU6" for example, then install 5.6 and rename that too, for example to "Airport Utility 5.6". Then you can rename AU6 again to Airport Utility, so the updates will go as normal again.
The older AE can also be managed by Airport Utility 6 afaik (I updated the FW of the old one with AU6, instead of AU5.6).
I have just completed firmware upgrade on my Airport Extreme and two Airport Express extenders. I received errors during upgrade on all three, but reading around on the internet I was expecting that. Apparently this upgrade could have used some more testing before being released to the public.
I'm not yet convinced it is possible to upgrade using the the newer (version 6.2) utility to upgrade older hardware.
I have not yet been able to get everything to run as it should.
1) Airport Extreme Base station 802.11n, first generation.
2) Airport Express extender 802.11n, first generation.
3) Airport Express extender 802.11n, 2nd generation
All Airport devices are running in bridge mode, my DSL router does DHCP so the Airport devices just provide wireless networking and access to a shared USB drive. The Airport Utility I used to upgrade the devices is version 6.2 on an iMac connected to the network through an ethernet cable.
I started by upgrading the basestation. This completed with an error: "An error occurred during upgrade of firmware -6727". The base station was no longer visible on the Airport Utillity. I first restarted the Airport utlity whcih did not help, then restarted the base station. I appeared on the utitity again, and I was able to connect wirelessly to the network.
I then continued to upgrade firmware on the extenders. Both the upgrades there also completed with the error "An error occurred during upgrade of firmware -6727". I restarted all devices in all kinds of different sequences; base station first followed by extender, and the other way around. The result is always the same, the devices show up briefly in the Airport Utility, the after a few minutes only the extenders show up and the base station is no longer visible.
I can now only connect to the wireless network if the extenders are not connected. The minute I connect one of the two extenders, the base station becomes unavailable. Even without the extenders, the transmission rates have dropped dramaically from around 300 to barely breaking 100 so I'm open to any suggestions how to proceed.
To fix my immediate issues, I rolled back the firmware on the Airport Extreme Base Station to 7.6.1 after which I could use the extenders again. I can only conclude that version 7.6.3 firmware does not work with this device if there are extenders connected. If I am wrong in this conclusion I hope someone enlightens me.
No problems here, same network. I did the FW updating the other way around:
First I updated Airport Utility to 6.2.
Mac connected to Airport Express number one, wirelessly, my side of the house;
FW update of the Airport Express number two on the other side of the house, then FW update of the Aiport Express number one that the mac was wirelessly connected to, then the central router TimeCapsule.
No problems at all.