What is the best way to trial changes before publication?

I want to upgrade my website: BL4 to BL5; add a shopfront; add a slideshow to index page. There WILL be snags and problems along the way (my capability, rather than BL issues!).
So how do I modify my old website and trial new things as a draft whilst leaving my existing site in tact and fully functioning? Then how do I seamlessly switch to the new version of website?

Upgrading to BL5 shouldn’t change your existing BL4 site.
To try out new features you could just add new pages without adding them to the navigation menu.
You can also try out new album features without displaying them publicly by checking “Hide from Album Set” and “Hide from Search” in the album’s Privacy tab.

After you’re happy with the design, you can just switch templates in existing pages, album sets, and albums.
If you’ve designed a whole new Home page, Just edit its properties an assign it as the index page.