Dear Nirakar,
Go to the Assingment where your Plant is assign to company code 1000 and select that assignment and delete it. Save. Again go to New entries and give your company code and plant and save. System will accept your assignment now.
But recomanded, when you are coping the objects do not do this because there are some other objects also linked with this settings. In future you may face other issues again.
If you like please give the points.
Thanks.
Inthiyaz