Definitely only do it on 1 or 2 at first to validate, but I think it's as easy as taking your balance down to zero and then re-entering with the corrected cost.
So if you have 10 widgets at $1.50 a piece in the system now, do an Inv Adjustment for -10 to clear the balance. Then do an Inv Adjustment for +10 at the corrected price.
You'll have to check the costing rules to make sure it will treat the positive Inv Adj as the "last" transaction. If you're not aware, inventory transactions on the same day may be ordered differently depending on the costing rules.
If it doesn't treat the positive one as the later transaction, you might have to do the transactions on different days.