Dash 8 200/300 and Q400 are a single (common) type rating under JAR, at least ('DHC 8'). JAR-OPS requires alternating recurrent checks on each variant of the type. No problem flying both variants, as long as you're paying attention!
If it were not for the manufacturer's desire to common-type the Q400 with the 200/300, it would have been a fantansic aeroplane ... all the very irritating little quirks and systems oddities would not exist. The DHC 9 would have been an absolutely superb aircraft in all respects - technical, operational, environmental and economic. But, they wouldn't have sold any! At the time it came to market, 50-70 seat regional jets were the future, and the market was percieved to be turboprop operators moving up to 70 seaters.