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.