No - not at all. We're doing this right now and we've structured the change by using QDeclarativeView/QtQuickView within our existing QWidget architecture. Depending on how you want to structure your application, using QML can be as easy as converting a widget/dialog/form at a time and allowing the C++ code to drive the QML, or going for a full re-write with the core of the program in QML with C++ helpers bound to it.

QWidget isn't going anywhere. If "Qt Widget -> QML" is an unpleasant "complete rewrite of your app" then don't do it. Stick with QWidget.