Dynamic challenge

Hi,

I have a question that is affecting my project delivery :

I have below structure and i want to change the data type of addr field to f or n DYNAMICALLY during runtime.

TYPES : BEGIN OF st_demo,

reg_no(10) TYPE c,

name(20) TYPE c,

addr(20) TYPE c,

END OF st_demo.

DATA : wa_demo TYPE st_demo,

it_demo TYPE TABLE OF st_demo,

Can anyone guru in abap advice me.