Hi Sunil,
I am not sure but try to transfer the actual cost from lower wbs elements to other WBS elements/other cost objects and then re post them back to level 1 wbs element then check if system is allowing you to remove budget from lower level wbs elements and reset OPSX. I think if commitments and actual are removed from WBS then it can be possible to reset the BUDG status of wbs. Though I am not sure of it but I would suggest you to check it once if it is possible and let us know if it works to some extent.
Regards
Saurabh