Airport Express firmware update/downgrade

I have an AirPort Extreme creating the wireless network in my home. I also have two AirPort Express units running external audio systems. One of the AX's is pre-N and is running version 6.3. The newest AX is N-capable and is/was running 7.3.2, same as the AE. Rogue Amoeba tech suggested I drop back to 7.3 on the newest AX to eliminate an audio lagging issue on that unit alone. This did not help, however, now the AX will not upgrade back to version 7.3.2. Selecting upgrade via the notice I receive when opening AirPort Utility just spins and spins and finally gives an error message that something must be awry with the AX. It is currently working for audio streaming (albeit the lag after a couple of minutes) and is showing a green light. Does anyone know why the unit will not upgrade after being downgraded for a trial? Thanks in advance.
jjsea