the entity cannot be updated because it is read-only crm 2011

We have Quote Approval  through Approval Activities. On approval of an activity we are retrieving all Approval activity for the same Quote. If All are approved then we are going to mark the Quote Approval Status as APPROVED, this is done through a plugin and its working fine. But if the Quote is Revised then the same Code is not working and getting error (the entity cannot be updated because it is read-only).

below link says same issue but in java script. we are facing same error in a plugin.

https://support.microsoft.com/en-nz/help/2006788/error-the-object-cannot-be-updated-because-it-is-read-only-occurs-when

Any help appreciated.