Hi Sand Buddies,
If you are trying to change an import parameter structure then SAP will throw a run time error. But if the import parameter has 'PASS VALUE' check box ticked, then you will be able to modify the structure.
Reason behind this: PASS VALUE check will copy the import parameter values in to the structure name and hence you will be able to change it where as if you are directly trying to modify the structure ,then you are trying to modify at the reference of structure which is not allowed.