Your current pages/posts/content should remain untouched. The importer only adds new content – it tries not to touch existing content.
However, it’s easy enough to do a full site backup that you should definitely do one, just in case. It only takes a minute and should save a lot of frustration in case there is some sort of incompatibility somehow.
most hosting companies provide a backup option – which would be the easiest option.
Might also be worth seeing if they provide a Staging site option, this will allow you to make changes to a ‘duplicate’ of your site before pushing those changes to the live site.