Is it permissible to change the intention of a voluntary fast—such as the fast of Ashura—from one day to another if an error is discovered in determining the day?

Changing the intention for a supererogatory fast to another supererogatory fast is permissible, as there is no text that prohibits it. Furthermore, a supererogatory fast is valid with an intention made during the day according to the majority of scholars, so it is even more fitting that changing the intention from one supererogatory fast to another is valid. If one intends to fast the day of Arafah or Ashura, or intends any other (supererogatory fast), they will receive the reward for those days, because the objective is simply the existence of fasting on them. Therefore, your fast is valid, Allah willing.