The only possibility I can find in the very comprehensive list of RAF trades on RAF web (a subscription site) is Draughtsman or Draughtsman Mechanical both of which are in the same trade group. Whether that makes sense in terms of his previous years as a fitter/armourer I am afraid I don't know. What is certain is that of the many trades listed, those are the only two beginning with D that make any sense
Sorry not to have a definitive answer, I'll enquire elsewhere while hoping that someone else could comment on a fitter/armourer changing trades.
MaxD