Bunkmaker,
several reasons for not upgrading SO's on the 400.First of all not enough training sectors(min 40 sectors for JFO upgrade) and it also means that the company has to choose between previous jet experienced SO and the ones who had only flown bug smashers previously(basically out of seniority).
Then for Airbus FO's, you have ZFT and also minimum sectors of 8 for line training.
Happy New Year and Safe Flying