I've started a new novel in and started the first chapter, but now find that I chose the wrong template when I created the project. Is it possible to load in a different template over the top, or do I need to create a new project? I checked the menu and manual, but couldn't find anything obvious.
Project templates are basically just a collection of settings to start you off from a certain point. There's nothing magic about them: you can recreate any template from scratch yourself, and you can change any template into any other one just by changing the settings. However, because templates are just a bunch of settings, there's no automatic way to 'change' them once you've started.
So, you've really got two options here:
a) Start again: create the new project with the correct template, and with both project open at the same time, drag the work you've already done into the new project's binder. If all you've done is write a couple of chapters, and not made any big changes to metadata, that's probably the simplest way.
b) Look at how the 'correct' template is set up and try to recreate the settings. This will be more or less easy depending on how different the two types of Project Template are, and on how well you know Scrivener.
From the sound of it, the first approach will be your best bet at the moment.