> If I have started development of a project using the open source version (LGPL), can I later purchase a commercial version of Qt and move my code under that license?
This is not permitted without written consent from The Qt Company. If you have already started the development with an open-source version of Qt, please contact The Qt Company to resolve the issue. If you are unsure of which license or version to use when you start development, we recommend you contact The Qt Company to advise you on the best choice based on your development needs.
https://www.qt.io/faq/tag/qt-commercial-licensing?hsLang=en