Try creating two Stock Adjustments: one to ELIMINATE all items with the wrong MAC, and another to put back the items BUT WITH your desired MAC as the COST.
Lets say you sell “Red Shirts”, and their current (wrong) MAC is 0.
Write down the Stock on Hand of Red Shirts [let's be random and say it's currently 2000 pieces]
Do a Stock Adjustment for Red Shirts to eliminate all its Quantity [in this case, -2000]. You're basically wiping out your stocks, so remember to use 0 (zero) Cost.
Do another Stock Adjustment for Red Shirts and use the original value , and this time write the correct COGS/MAC value you want in the Cost field. In this step you're simply putting your stocks back, BUT adding the correct MAC.