Redesigning a site without changing the original look

Looking to completely redesign my site but behind the scenes and once complete switch it out. How do is this done? Seems when choosing another site layout it requires immediate change.

The safest way to do this is to create a new free trial to build the redesign in. 

If there are large amounts of content from the old site that need to be in the redesign you can use @michaeleparkour's chrome extension to copy content from one site to another. https://www.squarewebsites.org/squarespacewebsites-tools-extension-pro

Once the redesign is ready to go live you have a couple of options. 

1. Disconnect domain from old site and reconnect it to new site. 
2. Temporarily password the old site, delete its content and use the chrome extension to copy the content for the new site into place. You'll also need to copy across code injections,  site styles settings and custom css. 

Which of the above you prefer to do will be influenced by the following factors:

• If your current site is commerce enabled option 2 is preferable because otherwise you lose customer information and sales history
• Do you have a large amount of your subscription period to run? If so, option 2 is again preferable because it runs on your existing subscription. Note that Squarespace Circle members can get partial refunds for early subscription cancellations. 

Option 1 involves less work and, in my experience, switching the domains over is very fast. Most of the DNS settings are already in place so you typically only need to change one CNAME and everything kicks in.