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If I add a module to a project by selecting New JFrame, but later alter the code of the class from 'extends javax.swing.JFrame' to 'extends javax.swing.JDialog' (and adding the arguments to the constructor, and invoking the JDialog constructor super(parent, modal) so that it works as a JDialog), the Inspector window still shows the top level entry as JFrame rather than JDialog. I have had occasion to do this when I realized after the fact that I need a modal screen, but did not want to rebuild the screen layout already done in a JFrame module.
if you do such changes, you have to reopen or reload the form class. I tested it in my environment and it works. But don't forget, that the e.g. value of method defaultCloseOperation should be changed. Then the top level node is changed to jDialog