Well I gave it all a try. Upgraded the three 5th gens, then rolled them back and upgraded again. They seemed to work for about 15 minutes then things started to go unstable. The primary started giving errors about double NA'T and handing out DHCP and the cable modem was rebooting like crazy. When it was working the speed test app is all over the place to, never stable and some times it hangs and if it does that you have to toggle wifi on and off to get Internet to come back on that device. All three routers act the same. Tested with MacBook Air, iPad mini, IPhone 5 and Latiude e6220 with 6300 intel wifi chip. I rolled the all back for now to 7.6.1 and Cable is now stable, routers are all stable, no more hangs and weird stuff. My speeds area ll back to normal and my shared drives and wifi printer are back as well.
Same problems here on an older Airport Express (802.11n first gen). This Express rarely sees the light of day as I only use it as required when traveling. I should have known better than to update while on the road.
I expected it to need an update so allowed 7.6.1 to update to 7.6.3. It downloaded just fine but after restart Airport Utitlity showed a green light for the internet but took a very long time before ever displaying a green light for the Express itself. It would delay as it was "reading" the Express.
Internet connection was very slow if it connected at all. I verified the internet was working by jumping on the hotel free wifi and no problems with a Macbook Pro or iPhone.
Restarting MacBook and Express did not help at all. iPhone seemed to be doing as well as it had before, but MacBook internet response was still almost none.
Used Airport Utility on my iPhone to downgrade back to 7.6.1 and things seem to be back to normal.
I'm not sure I still don't have a problem with the new Airport Utility 6.2 on my MacBook. The Airport Utility on my iPhone shows green internet and green Express. While the MacBook AU shows a green internet but a yellow warning triangle next to the Express complaining that it is off the network and do i want to "forget it"?