You can do that using the REPLACE function of MySQL
In phpMyAdmin type:
UPDATE w3t_Posts SET B_Body = REPLACE(B_Body, 'old setting', 'new setting');
Make sure that the old setting is unique. For instance in a forum that I did this we had everything under ubbthreads/ whereas now it is under forum/ so the statement was:
UPDATE w3t_Posts SET B_Body = REPLACE(B_Body, 'ubbthreads/graemlins', 'forum/graemlins/');
I hope this helps.