I think you could convert the careers, but you'd have to do something like linked professions, with gates for entry into new ones.
For example, a Thief could exit into Burglar, Montebanc, or Dancer with certain minimum skill bonuses and (maybe) stats and a minimum level (although the minimum skill bonuses might take care of that). Once they switched, their DP costs would change to the new profession. You may have to reset levels, or simply ignore them and base everything on having completed a particular minimum skill level "package" to exit to a new career.
You may have to create some new professions to mimic the WH world structure, but I think it could be done.