Hi Dori,
while not supported, this can be done. I've done it before for other bulletin board folks.
1) test this!
2) take good backups
3) the trick is to get bulletin board and sde.default to be at the exact same state prior to re-parenting. This is achieved by posting and reconciling the in the appropriate order.
4) once they are at the same state (query sde.versions), you can update the attributes in sde.versions for the version which you'd like to reparent.
UPDATE sde.versions SET parent_name = 'DEFAULT', parent_owner = 'SDE', parent_version_id = 1
WHERE owner = 'CHILD OWNER' AND name ='CHILD VERSION NAME';
commit;