Adding new theme. Quick question

So i’ve had my current theme for a couple of years. I’ve decided to install a new one that is responsive and has more features. I’m familiar with Theme Test Drive as a means of trying it before flipping the switch but I don’t believe with theme test drive I can customize the theme without interrupting the site.

I am curious as to the easiest way to install, customize and then migrate the new theme to the current site?

I’m thinking import my data to a new folder (mydomain.com/test), install WP + new theme there, complete the customization then literally copy/paste the theme folder(s) from the /test site to the /root site?

Is this the easiest way or is there something i’m missing?

Thanks in advance