Howdy @plinan, our suggestion for that situation is to write a condition in your comments.php template so that it uses WordPress comments for posts before the date you switched, and Vanilla comments after that date. It's a lot simpler than trying to migrate all previous comments.
We do offer importing services, but it wouldn't be free since we don't have a standard exporter for WordPress comments yet.
Comments
We do offer importing services, but it wouldn't be free since we don't have a standard exporter for WordPress comments yet.
It could look something like this (where '1 august 2011' is replaced with the day you switched), probably in comments.php in your theme.