but what about all my readers? Won't they still see it?
Yes, unless they act to disable it.
Won't that kill or discourage readership?
Yes.
I paid for the custom domain name but WP won't support SSL on it!? Terrific.
Me too and for two blogs but this factor was not included in the support docs at the time and still not included here in Important Notes Before Upgrading
https://en.support.wordpress.com/domains/#important-notes-before-upgrading