Hi, @JohnPMills. As long as there is an existing SSL certificate for that domain on another site on that team (and there is), that certificate will automatically be used for the new site on the same team. The SSL will work immediately.
I did check and both sites are on the same team. About avoiding any downtime when making DNS changes, this blog post is our best resource:
It recommends uploading a third-party SSL certificate but that is only if you are moving a new domain to Netlify and don’t want the SSL delays. Again, as you have working SSL on an existing site for this team and the same domain name, you won’t need to worry about that.