26 CFR § 601.505 - Revocation, change in representation and substitution or delegation of representative.
(a) By the taxpayer -
(1) New power of attorney filed. A new power of attorney revokes a prior power of attorney if it is granted by the taxpayer to another recognized representative with respect to the same matter. However, a new power of attorney does not revoke a prior power of attorney if it contains a clause stating that it does not revoke such prior power of attorney and there is attached to the new power of attorney either -
(i) A copy of the unrevoked prior power of attorney; or
(2) Statement of revocation filed. A taxpayer may revoke a power of attorney without authorizing a new representative by filing a statement of revocation with those offices of the Internal Revenue Service where the taxpayer has filed the power of attorney to be revoked. The statement of revocation must indicate that the authority of the first power of attorney is revoked and must be signed by the taxpayer. Also, the name and address of each recognized representative whose authority is revoked must be listed (or a copy of the power of attorney to be revoked must be attached).
(b) By the recognized representative -
(1) Revocation of power of attorney. A recognized representative may withdraw from representation in a matter in which a power of attorney has been filed by filing a statement with those offices of the Internal Revenue Service where the power of attorney to be revoked was filed. The statement must be signed by the representative and must identify the name and address of the taxpayer(s) and the matter(s) from which the representative is withdrawing.
(2) Substitution or delegation of recognized representative. Any recognized representative appointed in a power of attorney may substitute or delegate authority under the power of attorney to another recognized representative if substitution or delegation is specifically permitted under the power of attorney. Unless otherwise provided in the power of attorney, a recognized representative may make a substitution or delegation without the consent of any other recognized representative appointed to represent the taxpayer in the same matter. A substitution or delegation if effected by filing the following items with offices of the Internal Revenue Service where the power of attorney has been filed -
(i) Notice of substitution or delegation. A Notice of Substitution or Delegation is a statement signed by the recognized representative appointed under the power of attorney. The statement must contain the name and mailing address of the new recognized representative and, if more than one individual is to represent the taxpayer in the matter, a designation of which recognized representative is to receive notices and other written communications;
(ii) Declaration of representative. A written declaration which is made by the new representative as required by § 601.502(c); and
(iii) Power of attorney. A power of attorney which specifically authorizes the substitution or delegation.