@dennis-spillmann,
Sorry for the late response.
Below are provided the keys you should use to link the old connections:
POSTMETA
- _wpforo_crossposting_forumid
- _wpforo_crossposting_topicid
- _wpforo_crossposting_disabled = 0
COMMENTMETA
- _wpforo_crossposting_post
- _wpforo_crossposting_source = blog
In the POSTMETA COMMENTMETA tables, you should add the keys above with according values for each blog post/comment.
Please don't change the values provided in the codes (red-colored values). Just add the missing values. The missing values you should get from the forum, for example, Topic ID or Forum ID.