Inventory Type and Posting setup

Dear All,

I am still learning NAV since its new to me.  My situation as follows on the new company.  All inventory items has been setup as Average and unticked with post to GL on items setup.  This is the first month after migration and posting done on production order and received items on purchase orders for first 3 weeks and at the moment we wanted to change the inventory type from average to FIFO and tick the box post to GL automatically.  Can you please let me know what will be implication if we do this now or suggest me the best way to achieve what we want.