The first question is do they even make another 200 amp breaker for you 200 amp panel. If it's a load center I'd say does not exist.
If it's a panelboard you might be able to get a sub feed breaker but what is the point? It's allready covered by the 200 in the main panel. Double lugs would be cheaper.
Before you start you really nead to figure out your loads. Then you know how big of a panel you need in the garage, if a 400 amp service is large enough, & if you can tie into the 200 amp feeder.
200 amps sounds like it's on the high side for a garage turned into living space. How many sf 1200?
Myself I like to put the larger loads in the first panel. Not saying you can't sub a sub just that it does not seem ideal. But like I said we don't know the loads and how much of the 200 is used.
Tom
