You can change item unit only at the beginning of f/y through the option of check unit change in item master,
To modify units of measure for an item in item master in BUSY Accounitng Software you can follow these steps:
Step 1:Go to administration then click masters then open the item master in modify mode whose unit you want to change, there is a option is check unit change click this option then click OK to continue there after you can change your item unit,then save, Make sure it will change in your all transactions.
Step 2:If last year is existing in your company and item has been used, so you cant change the unit in item master,in that case you have to delete the item where its already used after that you can the change unit.