When I moved this blog from Blogger to WordPress platform recently, I couldn’t import the existing Disqus comments due to changes in the permanent link(URL) of posts. Disqus comment threads are based on the URL of the post.
What are threads? Threads are uniquely associated with each page which has Disqus embedded, typically an article or a blog post. For each of those pages, Disqus creates a thread that contains all of the relevant comments. Because a thread is associated to a page’s URL, or web address, they can sometimes become disassociated when the URL changes. If you’ve changed your website’s domain, you can use these new tools to restore these associations.[via]
So if the post URL changes, old comments will not be shown for the new URL even if we are using 301 permanent redirect for old post URLs. I have contacted the Disqus support team regarding this issue and they replied that a new tool is being developed to solve these problems. Surprisingly Disqus team introduced this tool last day!
Thanks to Disqus.