To Act According to Previous or Actual Taqlid

Q: A person was ignorant about certain rule regarding purity — during his following of a particular mujtahid — which caused invalidity of his prayers but he discovered his mistake after he changed to another mujtahid in taqlid. Now, should he act upon the fatwa of the previous mujtahid or the present one?

A: If the practices he was doing, which he considered as void, were invalid according to the previous taqlid but in order according to the contemporary one, he may consider the contemporary taqlid and he is responsible for nothing then.