Question about Type Ratings

[SGAS]

If a airframe maker certifies a new type, a derivate of an existing aircraft, lets say Fokker F28 to F100, or 737-200 to 737NG. What is decisive to have this airplane covered under the same Type Rating as the original plane?
Is it the the handling, the systems or the flightdeck layout?

I now that the classic 747's for exemple have another rating than the 747-400. Are the systems that different, or is the new 2 crew flightdeck the decisive factor?

The 737 on the other side is under the same rating (with differential courses) in all its versions, aren't 737-200 and say, 737-900, quite different in handling and systems?

Or is it just a matter of how well the manufacturer goes along with the certificacion agency ;)

I hope you understand my question, I speak two langauges perfectly, sadly english is not one of them :\

[buttline]

In the UK, the 737-200 rating is a completely separate rating from the 300-900.

The 300-500 is known as the Classic. The 600-900 is the NG. You can do a type rating on either the Classic or the NG but to be rated to fly both, you must do a one week differences course (roughly 2k pounds) in the other type.

I seem to remember someone telling me during the training that for a type rating to be considered 'common' among variants there must be at least 50% of the systems unchanged.

[Gin Slinger]

Under JAA -200 and -300/900 require different Type Ratings.

Somebody told me that historically the -200 and the Classic could be operated in the UK under the same Type Rating, but the CAA discontinued this in light of the Kegworth accident - the very different instrumentation system fitted to the two types was considered a factor in the good engine being shut down.