When we moved t from the old forum, we had to copy-paste all old posts into the new forum, which means, that links within the posts, might not work anymore correctly and the person, who wrote the original post, won`t see your replies. However, you can tag anyone (with @), to make sure, they see your new comments and come back to you for a reply.
Note also, that links related to user profiles and tags won't work anymore, even though the user is already signed up here by the same name. You can track em down by searching the "Members" or, as Mr Hoenig suggested, tagging them in a reply.