Cannot modify AP Invoice distribution code combination for Tax line

Hi,

I am unable to update the tax account combination for a tax line, scenario is I have entered a manual AP Invoice and entered an item line with tax classification code populated. After tax line is generated when I go to distributions the tax line account combination is greyed out and I am not able to update.

I can update code combination for a tax distribution in 11i, in R12 is there any option or configuration which can give me option to update the code combination

Thanks,

Rama