Hi there,
Thanks for writing in! It is very important to note that while the importer will not overwrite or alter any of your posts, pages, or portfolio items, however your Customizer settings will be altered upon setting up a demo (this is necessary so that we can setup your installation similar to the demo). If you already have your website setup but are still curious to try out this feature, please make sure that you first backup your Customizer settings by going to the Customizer Manager page under X Addons section.
Hope that’s clear.