If you were using BB Press 1.x with the BB Attachments plugin it can be a bit of a challenge to re-attach them to BB Press 2.0 in Wordpress using the GD bbPress Attachments plugin.

Here are the steps I took to accomplish this migration.

1. Run the BB Press migration process making sure that both the original BB Press 1.x tables and Wordpress tables are in the same database.
2. Download the original attachments from the bb-attachments folder.
3. Switch Wordpress to not use year/month upload folders under Settings – Media.
4. Upload the attachments to the wp-content/uploads folder.
5. Install the GD bbPress Attachments plugin if you haven’t already.
6. Run the following MySQL script

INSERT INTO wp_posts
(post_author, post_date, post_date_gmt, post_content, post_title, post_status,
comment_status, ping_status, post_name, post_modified, post_modified_gmt,
post_parent, guid, menu_order, post_type, post_mime_type, comment_count)
SELECT replies.post_author, replies.post_date, replies.post_date_gmt,
SUBSTRING_INDEX(filename, '.', 1), SUBSTRING_INDEX(filename, '.', 1), 'inherit',
'open', 'open', CONCAT(bb_attachments.id,'.',SUBSTRING_INDEX(filename, '.', 1)),
replies.post_modified, replies.post_modified_gmt, replies.ID,
CONCAT('http://nonamers.org/wp-content/uploads/',bb_attachments.id,'.',filename) as guid,
0, 'attachment', mime, 0 FROM `bb_attachments`
JOIN bb_posts ON bb_posts.post_id = bb_attachments.post_id
JOIN bb_topics ON bb_topics.topic_id = bb_posts.topic_id AND bb_topics.topic_status = 0
LEFT JOIN wp_posts as topics ON topics.post_status = 'publish'
AND topics.post_type = 'topic' AND topics.post_title = bb_topics.topic_title
LEFT JOIN wp_posts as replies ON replies.post_parent = topics.ID
AND replies.post_status = 'publish' AND replies.post_type = 'reply'
AND replies.post_author = bb_posts.poster_id
AND replies.post_date_gmt = bb_posts.post_time
WHERE bb_attachments.status = 0 AND replies.ID IS NOT NULL

Tweak any attachments if necessary.