Yes, your client will need to sign up for WordPress hosting first so that you have the site ready to configure.
Once WordPress is setup and configured to your liking, you'll install your theme and any plugins that you want to use. What you could do is have her GoCentral site open in one window and the new WordPress install open in another. Then go page by page and copy-n-paste from one to the other until all the content is moved over. You can then tweak as needed.
I don't work for GoDaddy so I'm not aware of their refund policy. However, once you have the WordPress site setup exactly the way you want it, you'll then have to move her domain name from GoCentral to the new WordPress site.
Once the domain is pulling the new site, I always wait a week to allow the new location to propagate across the network. Then you can cancel GoCentral and determine if a refund is available.
Judith "All people are the same; only their habits differ." ~Confucius