Manual Relink of Organizational Identity fails because co_person_id is frozen

Manual relinking of an Organizational Identity from one CoPerson record to another CoPerson record is broken because the co_person_id field on the CoOrgIdentityLink model is frozen.

An example series of operations is
*GET https://registry-test.cilogon.org/registry/co_people/relink/1692/linkid:408/tocopersonid:1321*
*POST https://registry-test.cilogon.org/registry/co_org_identity_links/edit/408*

The POST returns a 500 Internal Server Error and the flash message says "co_person_id" is frozen and may not be changed.