As I understand it, Qt is released under a mix of GPL and LGPL, so as long as you release the source code of your application you should be fine.

I'm not sure how they can prevent you from transitioning from an open source license to a commercial license? You can develop your app for a year either as open source or privately without releasing anything, and then decide to purchase a license? Not sure if I'm missing something, but how does could they prevent this (legally or practically)?