I’ll be needing to do this soon. I need to allow one client to start creating content for writing a book, before finally deciding on the title and therefore the domain name later! Once that is finally decided we can register the domain and then create a new site and shift only the content across.
The other alternative is to get a hosting company to shift the site across to the new domain and it is a fairly cheap step to do this, I recently asked my favourite host company Siteground to do it and I was very pleasantly surprised and how fast, easy and reliable the service was.
Here is an excellent article on how to copy all your content from one WordPress website to another.
It’s from 2014 so not too out of date, hopefully most of the menu steps will be pretty similar.
Read more about the WordPress Importer.