It’s always better to run a fresh import if you’re intending to import the whole content.
However you may also import particular contents if you want to integrate them with your current arrangements.
Importing doesnot destroy the contents but they may override the present contents , you need to keep in mind that importing the demo data will overwrite your current settings. It’s always better to take a full backup of your website if you’re in dillemma or start a fresh installation and import. Here is the reference for backing up your website.
Hope you get it. If you have any confusion please let us know.