Using the following process the following code fails.
Setup:

First create a container QWidget with HBox layout and two buttons.
When the first button is clicked, switch to VBox layout.
To switch layouts:

1. Create a list of child widgets and remove them from the current layout.
2. Create a new layout.
3. Add the widgets from list to the new layout.
4. Set container widget to new layout.
   Note:
5. setParent(None) for children is ineffective.
6. setLayout(None) for children is invalid.
7. del old layout is ineffective.

Pseudo code test program: s=self, w=widget, wl=widget list

    import sys   
    from PyQt5.QtWidgets import *  
    class Lw(QWidget): 
      def __init__(s):
        QWidget.__init__(s)      
        s.lay=QHBoxLayout(); s.setLayout(s.lay) 
        s.setGeometry(50,45,220,220)  
        bu0=QPushButton(); bu0.setText("Switch to VBox"); s.lay.addWidget(bu0)
        bu0.clicked.connect(s.rslay)  
        bu1=QPushButton(); bu1.setText("Dummy"); s.lay.addWidget(bu1)
        s.show()
      
      def rslay(s):  
        wl=[] 
        while s.lay.count()>0: 
          w=s.lay.takeAt(0).widget()
          wl.append(w)
          w.setParent(None) # Ineffective  
          #w.setLayout(None) # Invalid  
        #del s.lay # Ineffective
        s.lay=QVBoxLayout() 
        s.setLayout(s.lay)  
        for w in wl: s.lay.addWidget(w)  
    
    if __name__== '__main__':
      app=QApplication(sys.argv) 
      lw=Lw()
      sys.exit(app.exec()

Program will error:
QWidget::setLayout: Attempting to set QLayout “” on Lw “”, which already has a layout

The child widgets original layout is not dissociated – even if the layout is deleted or its parent is set to None.