Hi Kim - 

Absolutely. You can connect a new domain to your nation at any time, disconnect your website from your current domain, and then connect the new one. 

You can disconnect your current domain under Settings > Domains > [your custom domain] > Sites > [URL] > Remove subdomain. 

To connect your website to your new domain, follow the same steps until you're on the Sites page. Then click the "Attach one here" link and input your desired subdomain (like 'www') into the URL field. Pick the website you'd like to connect it to from the "Use URL for site" dropdown menu and click save.