Hi Niru,
Something else to keep in mind is to check whether this field is configured to be changeable or not. Confirm by using transaction FB02 that you are able to change it manually. If not, I expect the FI_DOCUMENT_CHANGE will not accept the change.
What type of line items are you trying to change? I think FI_DOCUMENT_CHANGE only supports changes to customer and vendor lines; and only specific fields (not including KONTL) for those. I'm not sure, that's based on doing a quick look at the code in the function.
If the function module call gets too complicated, you could always go old-school and do a Call Transaction to FB02.
Jim