Not able to change plan value in appropriation request

Dear Gurus

I am not able to the change the investment cost in plan value tab after creating an appropriation request. We have done an upgrade from ECC6 to EHP6. Earlier we were able to change. We want to change the plan value ,since an approximate value is entered while creating and later, on we change the value to the cost incurred.

Please help

Regards

Vidhyadharan