in doing a project for a client, I was tasked to create a new Shopify store for their current WordPress store (powered by Foxycart).

I’ve completed the Shopify store now, but the thing is, my client still wants to keep the WordPress site active and assign it a new domain (via GoDaddy) after I assign the current domain to the Shopify store.

How do I back up the old store properly so that I can keep all its info intact?

I tried this guide but I was unable to find a wp-config.php file in the WordPress site. Apart from that, changing the domain via the functions.php file with this code did not work.

update_option( 'siteurl', 'https://example.com' );
update_option( 'home', 'https://example.com' );

Also, how can I possibly change the domains in a smooth manner without affecting SEO?