What is the ruling on changing the intention of a volunteer performing Hajj on his own behalf to performing Hajj on behalf of his deceased mother who was unable to perform Hajj?

If you have performed the pillars of Hajj and nothing invalidating it was issued from you, then your Hajj is valid, and the doubts that come to you after completing it are of no consequence. This is because the default is the validity of the act of worship until its invalidity is proven, and not every flaw in Hajj invalidates it. Therefore, it is permissible for you to perform Hajj on behalf of your deceased mother, and for that, you will have a great reward.