How to change the main site theme? should change at main site or set a sub site to new them?

Dear Sir

Q1

I have multi-blog theme at main site new, I want to change to new theme.

http://preview.themeforest.net/item/amilia-multipurpose-one-multi-page-wp-theme/full_screen_preview/13841964?_ga=1.110255813.1995049209.1470013385

amilia-multipurpose-one-multi-page-wp-theme,

or upfront spirit theme

How to change the main site theme? should change at main site or set a sub site first to new them? If set at main site, then main site will broken, if set at sub site, main site have hundreds pages new. is need re do it at sub site? How I should do to change a new theme?

Q2

Is upfront spirit theme can build same as amilia theme ? which theme is more easy?

Your are very helpful,

Thanks!