This depends on the wording of the agreement and whether both parties agree that the terms of the agency relationship should be changed.  If this is the case, the original written agreement must be complied with in terms of any requirements for making amendments to the agreement between the parties.  If the written agreement does not provide for amendments, it will be necessary for all parties to the agreement to consent to the amendment and for a further document or addendum to the agreement, to be agreed and signed, in order for the amendment to legally take effect.