Make the populated Proposal Creator field in Proposal Grid Static

The Proposal Creator field in the Proposal Grid is populated based on the log-in credentials of the user who creates a proposal. However, if the user is subsequently removed from the User list, all entries of that user in Proposal Creator field are deleted. Ergo, the history is lost. Would like an enhancement that once the PC field is populated upon creation of a proposal, the field becomes static so that the history of the user who created the proposal is not lost.