Hi Siva,
this error (The messge MEPO 087 "Plant can no lognger be changed") is introduced by the following note which applied in your ECC system:
441759 - Return indicator and reference docs changeable after GR/IR
This is to avoid inconsistency after follow-on documents are created for SRM PO.
If you want to change this, you have to use BADI or customizing way to convert the error message to warning.
Best regards,
Ivy