Changing existing template to "Custom"

Hi, I wondered if someone could help me out in the start of my site redesign. I'm moving from a Wells-based portfolio site to something more 'one-page scroll'. I notice that if I start from scratch with a new site I can enter 'Custom' when the wizard asks me what I want to use the site for, but does anyone know how to change my existing site template to Custom? I know it's based on Brine, but I'd like to use the Custom formatter to create the site structure as it seems a lot easier.  

I don't believe you can go through the custom process when switching templates withing the same site. 

There are a few options. 

First create your new site as a completely fresh project, using the Custom workflow. 

When you're happy with your new site either

• Create s subscription for the new site
• Disconnect the domain from your old site
• Connect the domain to your new site
• Cancel the subscription for your old site

OR

• Temporarily password your current site while you perform updates
• Delete / disable the pages of the current site
• Change the template on your current site to the same as the trial
• Use @michaeleparkour's Chrome extension to transfer the pages from your trial site to your current site
• Copy across any custom code, Style editor settings, and rebuild the header and footer on the current site
• Test
• Remove  the password so that users can see the new site