costing method

Hi, we have items with uncorrect Costing method.
We would like to change the costing method, but navision hinders us to do so,
because of entries in Item ledgery Entry.

it is a simple security programming in the validation of the "costing method"
in table 27 "Item"

we would like to outcomment this security programming, change the costing method and then activate the programming.

Are there anybody how has this done before?
Or what else can we do?
(create a new item is impossible)