Answer:
All the definitions for corrigendum refer to corrections to a document or printed material. That does not seem to be the appropriate term for your situation.
The following is general information. If the attorney-in-fact under a Power of Attorney has retired and can no longer serve in that office, the POA should be revoked and a new POA must be executed. A copy of the revocation should be given to any facility where the POA had been used in the past.