Hi Sand Buddies,
As @Heber Fabiano has pointerd out, please check to see if you are trying to change an IMPORTING PARAMETER. If you have mentioned the structure name in the interface, it means that it is an importing parameter to the smart form, and therefore cannot be changed.
Only EXPORTING parameters value can be changed (as in changed and returned back to the print program).
As @Eitan Rosenberg has suggested, Tick the "Pass By Value" option to perform the same.
Regrards,
George Abraham