How to switch to upfront seamlessly

Afternoon all

I need to switch to a responsive theme, I really like Upfront Sprit but I need to edit it on the site without causing havoc on the current site.

I've tried playing with it on a dormant domain but it's not helping to work out how it will work with my plugins (membership, directory etc).

I've tried migrating the current site to the dormant domain but I think that because its a multi site, it keeps crashing. (using import buddy).

How else can I activate and edit the theme without ruining the current site?

Any help gratefully received.