Making parameters field uneditable / editable at runtime in SAP ABAP

REPORT ztestr_screen_change.

SELECTION-SCREEN BEGIN OF BLOCK b01 WITH FRAME TITLE text-001.

PARAMETERS: rd_sel1 TYPE c RADIOBUTTON GROUP sel DEFAULT 'X' USER-COMMAND upd.
PARAMETERS: p_gui TYPE rlgrap-filename.
PARAMETERS: rd_sel2 TYPE c RADIOBUTTON GROUP sel,
p_app TYPE rlgrap-filename.

SELECTION-SCREEN END OF BLOCK b01.

AT SELECTION-SCREEN OUTPUT.

LOOP AT SCREEN.

IF rd_sel1 IS NOT INITIAL.

IF screen-name = 'P_APP'.
screen-input = 0.
ELSE.
screen-input = 1.
ENDIF.

ELSE.

IF screen-name = 'P_GUI'.
screen-input = 0.
ELSE.
screen-input = 1.
ENDIF.

ENDIF.

MODIFY SCREEN.

ENDLOOP.