As Google+ hosted comments become more common in our blogs, we see more questions about blogs recently renamed, and disappearing comments.
Blogs renamed (and URLs changed) from BlogSpot to BlogSpot, from BlogSpot to a custom domain, and from custom domain to custom domain - all become concerns, with blogs that use Google+ hosted comments. This starts with the difference between Blogger, and Google+, hosted comments - and how they reference the host blog.
Blogger hosted comments link to the blog - but Google+ hosted comments (aka Google+ stream posts) link to the URL. Any time you change the URL of your blog, you will lose Google+ hosted comments - from your blog. The Google+ stream posts, however, are still out there - and they still link back to your blog.
Any existing Google+ stream posts, mentioning your blog, will link to the previous blog URL, after you change the URL.
A properly executed custom domain migration will redirect the BlogSpot URL to the Domain URL. Any existing Google+ Stream posts will continue to link to the blog, under the new URL, because of the redirect.
Any other URL changes - BlogSpot to BlogSpot, custom domain to custom domain, or even custom domain back to BlogSpot - will leave you with the old URL going nowhere - and no way to redirect from the old, to the new.
Any Google+ comments, that are associated with your blog, are out there, in your readers Google+ streams. Any readers, who have a Google+ account, have already Circled you - and are Following your comments (and posts) in their Streams.
Your old comments (and posts) are not visible to people who have not Circled you, after a URL change - and they would not be visible with your posts, for people who have not Circled you, before a URL change.
The comments are still in your Google+ Stream, for you to see (and reply) - and they are still in your Followers Google+ Streams, for them to see (and reply). The only thing that is lost is your ability to see all Google+ hosted comments, conveniently, in one place.
The conversations between people, mentioning your blog, can continue - and you can participate - when you are conversing with people who have Circled you (or published publicly). You can't participate in any conversations that are started by people who have not Circled you (and are not published Publicly) - but you could not do this, with the blog under the previous URL, either.
In reality, all that you lose is the ability to associate all of the different conversations about your blog, between the people conversing about your blog. The conversations can still continue - as individual conversations. And you gain the ability for the world (a bit at a time) to learn about your blog, and how it applies to their lives.
It's like comment moderation under Google+ - you lose a tiny bit of control - but you gain visibility.
An Important Update
If you did not use a Blogger / Google account when you Followed this blog, years ago, you are probably not Following now . During the past...