Changing an Org from Standard to Average Cost

 I have inherited a business where the supply warehouse was incorrectly setup under standard cost (same as our productive inventory).  The organization is separate so how would we proceed with redoing the supply org to average cost?  Is it something that someone who has not done the setups can do by reading the manual?  How much work would it take?  Any information would be appreciated - we aren't sure if we should try fixing the problem or live with it and rig our own solution.  Supplies should transfer in our environment at what we pay for them - there is