Difference training is required for flying different models within a same type aircraft... as example - if you are qualified on 757, you would require difference training for the 767, as both ratings are issued as a single type rating... was the case of the 707 and the 720 as well.
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Difference training might be just a "few hours" in a classroom. As an example, when my airline leased a 747-341, our 747-287 pilots were required to get an 8 hrs difference training to study specifics of the other 747 model.
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Transition training is required from one type rating to a new type rating... as an example pilots flying 747-200 require the complete training for issuance of a 747-400 type rating, which is a separate rating. It is a training that requires 80 hrs classroom program, simulator training and line training.
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Happy contrails