i've found in XOOPS france this code to return to cbb (i thought)
INSERT INTO `[prefix]_bb_categories` (cat_id,cat_title,cat_description,cat_order) SELECT cat_id,cat_title,cat_desc,cat_weight FROM `[prefix]_d3forum_categories`;
INSERT INTO `[prefix]_bb_forums` (forum_id,forum_name,forum_desc,forum_order,cat_id) SELECT forum_id,forum_title,forum_desc,forum_weight,cat_id FROM `[prefix]_d3forum_forums`;
INSERT INTO `[prefix]_bb_topics` (topic_id,topic_title,topic_views,forum_id,topic_status,topic_sticky) SELECT topic_id,topic_title,topic_views,forum_id,topic_locked,topic_sticky FROM `[prefix]_d3forum_topics`;
INSERT INTO `[prefix]_bb_posts` (post_id,pid,topic_id,post_time,uid,poster_ip,subject,dohtml,dosmiley,doxcode,dobr,attachsig,approved) SELECT post_id,pid,topic_id,post_time,uid,poster_ip,subject,html,smiley,xcode,br,attachsig,approval FROM `[prefix]_d3forum_posts`;
INSERT INTO `[prefix]_bb_posts_text` (post_id,post_text) SELECT post_id,post_text FROM `[prefix]_d3forum_posts`;
i think it can works but losing something like comments, etc
