It may have been a while ago, but I added 300-900 to my 200 rating. It most definitely was not a full conversion! Whilst it's a different 'stamp' in your licence, it was a 'differences' course. Or that's how I would describe it.
How can it be a totally different type rating when you are allowed (if qualified) to fly both types. I remember well the times I would fly -200, -300,and -400 on the same day, I wouldn't be surprised if others have done the same!