As far as i know you need a differences training between the NG and the Classic. I have seen a company where it was a 4-sided A5 handout and a signature on a sheet that covered that, others required at least a simulator session and a few days of line flying under supervision. The different direction, from the Classic to the NG, it was a simulator session and 16 sectors of line training as well as a 4 hour CBT program.
Best to check with your employer what they require and then find an ATO that does the cheapest possible way of a difference training. In my opinion the induction course to the airline should be more than enough to cover the difference training as well but i have no clue if they have a program in their OMD for that.