How to change the direction of a relationship type with existing data
I agree with ErikH, but if you don't feel comfortable updating the database, you can...


create a new relationship type with the opposite order (b_a vs a_b)
do an advanced search, selecting only individuals with that
relationship
select the action to "export" the contacts
select two    fields (1)the internal contact_id and (2) the
relationship type field    with the internal_contact_id  and export 
(verify that you did this    correctly and the ids match as
expected!)
use civicrm import to    import the csv file to new relationship type
created in the first    step.


May more steps, but if you don't know how or want to mess with the database, this will work.