Are you unable to change your existing paper template to another one, are the paper selections grayed out? If so, do take note that for the built-in templates, you can select only the pages that match to the size of your existing page if there is content on the page. You can only change to any other template as long as there is no content on the existing page for built-in or the custom templates.
This is because we now support multiple page sizes in NS2, it is not possible to scale the notes already existing on the current page to a new sized page. Hence, we had to limit these options. You can however use the ‘Add new page from template’ option via the Plus icon to add a fresh new page template of any type without any such restrictions.