I’m not aware you can, you just create the correct kind of page you want and delete the old. This is probably to prevent people from accidentally converting a page and losing their formatting.

Actually, it’s pretty simple (usually) to create a new page type and copy and paste the data from one to the other.

We do see requests for the ability to change page style from time to time (via a command or some other such means). So that is something we may consider for a future version of NoteBook.