Answers:
Like earlier stated here, if you have 604 rating, 2 day differential course will do it for 605. Maybe possible during recurrent training, not sure though....
Then, with 604 rating you will need 10 day differential training to get CRJ100/200 (Challenger 850) type rating. 3+1 sims and rest of it is theory.... At least this is how it is with JAA.
And now the question:
Does CRJ100/200 have any short-cuts for CRJ700/900 rating?