Update from up in here in mid-2015:
We had two 23" SA550s to fix.
I had read awhile back that it needed to update the firmware via the HDMI input. When it came time to do the fix, I reviewed what's online today, seeing that some folks claim success with updating over VGA.
Using both a box and a laptop - one at a time - as hosts for the upgrade and testing, I began with SA550 #1.
The box used a VGA connection. The laptop had both VGA (via DP-to-VGA adapter) and HDMI.
Both machines acknowledged the SA550, but - except for an evanescent initial display from the laptop via HDMI, the 550 refused to display a signal from either machine.
The display's miserable "push" buttons did no good. Basically, they functioned, but - except during that first successful but abortive display session, the buttons refused to respond to up/down selections.
That was fun. I resisted the temptation to fire up the soldering iron for use as an unassailable heat source on those buttons.
Finally, on the laptop, I (for the nth time) ran the update via HDMI. This time, it actually displayed the progress bar, percentages, etc., and took 2-3 minutes to run.
Viola - success. Confirmed the result over HDMI and VGA, on laptop and box.
The second display did not succeed over VGA, but did succeed the first time over HDMI.
Conclusion: Don't waste time with VGA; use the HDMI input to perform the firmware upgrade. I also believe that the pre-upgrade firmware is hosed in a way that lends itself to inconsistent symptoms.