How to Apply Panelizer on already created nodes of a particular node type?
The module's lead maintainer, merlinofchaos, responded to this on drupal.org; in short, it's only possible if using the IPE, there's an option to make the changes global i.e. affect the default configuration, otherwise the changes have to be made via the entity's Panelizer tab, e.g. admin/structure/types/page/panelizer.