adding a new theme to existing web or start fresh

I’m about to give my website a makeover. It’s a site with a lot of posts (more than 100). So I’m wondering which is best:

use the existing site (making sure the WP version is up to date) and then apply the new theme to it. Or,

start with a fresh WP version, add the new theme, and then import the posts and page content.

I would normally do the latter, but I’m always had trouble getting the library files to import over properly.

Thanks in advance!