You sure can.
In Stage 1, just return to the structure selection page, select a new structure and click 'Ok', I'm done'.
In Stage 2, just click on the SETTINGS tab on the left side of the screen and select the “Change structure” menu item to revisit your choice.
If you do change your structure, you may also need to change any content you have already added. This is because titles and content you created under one structure may not fit or make sense when based around another structure.