P3002 Copy BOM Function

Hello-

When we use the copy function for our BOM's to copy to other lines in a plant or other branch plants the UOM field populates correctly but when you enter the component you want and click off that field the UOM field changes back to primary UOM that is in the Item master.  We found a way in the processing options that would leave this field blank but what we would like to do is see if we can make it so this field does not change back.  Is there a way to do this? 

Also, where is the IXUOM pulling it's information from in P3002?  We tried to change the IMUOM1 in the Item master (P4101) and it didn't seem to take.  For some reason it seems to always change back to LB.