Update 26th October 2018
Looks like the article I linked to about migrating comments no longer exists. Disqus provides a number of tools to help with site migrations:
I’ve not been very active on this site this year - sorry!
I’m planning to migrate this site from drupal to hexo and
as a first part of that and also in an attempt to combat comment spam I’ve
moved all comments to disqus - we’ll see
if that helps.
I found the article Manually Migrating Comments from Drupal to
Disqus really helpful when working out how to export comments from drupal to
disqus.
For anyone else thinking of trying out Hexo, John Stevenson has a helpful set
of articles on his blog: http://jr0cket.co.uk/hexo/