There are actually two ways to change the currency decimal of the item; one can be accomplished using the GP's built in utility --> Microsoft Dynamics GP menu - Utilities - Inventory - Change Decimal Places. The other is by using SQL scripting:
Example: Update to 5 Decimal Places, the Update statement is:
Update IV10001 set DECPLCUR = '6' where ITEMNMBR = 'CURRENCY'
Note that
1 - 0 decimal place
2 - 1 decimal place
3 - 2 decimal places
4 - 3 decimal places
5 - 4 decimal places
6 - 5 decimal places