I bought a 3TB time capsule yesterday and connected it to my network easily, and as everyone else has found, realized that having updated my Airport Utility to 6.3 on my Macbook Air and our iMac, all 6 of my Airport Express devices set up to drive speakers in various locales were now invisible. The purported IPV6 "fix" from Apple was, not suprisingly, useless. I did multiple hard resets on every one, and of course, despite all logic to the contrary, 2 of the 6 eventually were seen by APU on my Mac Air. Multiple restarts of the Time Capsule, all manner of other workaround including hooking eack APE unit to the computer with ethernet were unable to get them recognized.
I tried multiple times to download an older version of APU but the OS doesn't let you delete the new one and install the old one. I know if I was more tech savvy there is probably a workaround for this as I seem to have read but it's a bit beyond what I am capable of.
So, my ultimate solution...we had an old iMac from years ago mothballed in the basement that I could never bring myself to clean up/sterilize the HD and sell but that we never need/use. Hauled it up, unpaired the mouse from the iMac, paired it to the old machine and got it all running, and in minutes the old Airport Utility had every Airport Express listed in the sidebar ready for configuration with the new netork. Ten minutes later every one is up and running.
The only glitch is that on the APU v. 6.3 on my Air and iMac "sees" every Airport Express unit, correctly named for their locations in the house, etc., but there is no green light/dot next to the icon. If I wasn't listening to music through them I would assume they were offline or not connected but I can use iTunes and iPhone remote app like always. My conclusion, for now, is not to even mess with the current disaster that is APU 6.3 until they (Apple) acknowledge it's a complete failure, unless I want to cough up another 400$ for new Airport Express units.
So perhaps readers here can locate an older version of APU the way I did.