I updated my HDX to 17.4 yesterday. I checked everything was 'Ready' on the network status and then went to the transponder setup. I was met with a red instruction, telling me to power cycle the transponder. I pulled and reset the CB. It did its thing and then everything came good on the status report after less than a minute.
As an aside, all the serial inputs/outputs worked normally.
As another aside, during the update I got a 100% report on the 'Screen something or other' just a short while into the update. It said not to turn off the power. After quite a long time the same screen was still there and nothing seemed to be happening. Eventually I turned off power and then turned it back on again. The system continued to update and all was well. I don't know if the 'freeze' is meant to happen or whether it was just me. All the time during the update my aircraft battery was being charged, so absolutely no chance of low voltage causing the 'freeze'. Just putting this out as information because, as I said, the update did continue normally.