What body exists to support the authorization of changes and to assist change management in the assessment and prioritization of changes?

The 'change authorization board' is not an official ITIL term - the correct ITIL terminology is Change Advisory Board (CAB).

In ITIL Change Management, the Change Advisory Board (CAB) is a group of stakeholders - typically drawn from IT, business units, and other relevant areas - convened to assist the change manager in evaluating the risk, impact, and priority of change requests. The CAB provides advisory input to help ensure changes are authorized appropriately and scheduled to minimize disruption.

The change implementer is the individual responsible for executing a specific change, not for advising on prioritization or authorization.

The change manager is a single process owner role responsible for overseeing change management, not a multi-stakeholder advisory body.