Ronin is almost right
The 200 and 300 are almost identical except for weight and seating capacity, I can't remember the exact figures. However they are identical flight deck wise and have a common type rating. It is the Q400 which is significantly different (twice the engine power, 100kts faster, 10 tonnes heavier) and which in JAR land at least requires differences training in the sim. However, all have a common type rating and can be flown concurrently if the airline has a mixed fleet.