If I change the unit on an item, will it change the unit in all the the existing voucher of that item?
Yes, if you change the unit of item master then it will change the unit in all existing vouchers of that Item. You can follow these steps to modify the item unit:
Step 1: Go to Administration menu section then click on master menu
Step 2: Choose Item then click to Modify.
Step 3: Select the required item master and click on check unit change then press ok tab.
Step 4: Now, change the unit and save the master.