H1B Petitions approved Changing LCA

Hi Everyone,

   My H1B petitions approved on 14-Aug-2012 but Now i have changed the project(client) so I need to Change LCA for new client so wanted to know when it can be done after stamping or before stamping  the visa.Do we need to pay any processing fee while going for stamping H1B at consulate. Thanks !!


H1B Guy.