As I said, it is pretty clear for me that it's not a safety flaw considering that as far as I know it was the only one crash related to that jackscrew and the cause was gross improper maintenance.
However, maybe I'm too dumb for this, but I still don't get it. Why did they asked changes for certain aircraft while almost all other use the same system? Are there some particular differences related to design on DC-9, MD-80/90 and Boeing 717 fleet over others?