Not really. The only thing you need to do it to walk fast when you get off TG flight in NRT. NRT is pretty easy to connect within the same terminal. Your chance of making 50 minute connection is very good. As Often1 posted, there are many factors affecting if a flight may be held or not. It makes no difference if you call them or not. I won't be concerned about F being oversold. UA F is usually full of non-revs who makes UA look good by filling up otherwise empty F cabin.