What should she do after exchanging her old gold for new gold, out of ignorance of the Hadith in which the Messenger of Allah, peace and blessings be upon him, prohibited exchanging gold for gold and silver for silver?

If the exchange was done with equal weights of the two gold items, then there is no objection. However, if there was an excess in one of them, then the exchange is forbidden, due to the saying of the Prophet, peace and blessings be upon him: "Gold for gold, silver for silver, and salt for salt, like for like, equal for equal, hand to hand." Narrated by Muslim. The sale must be annulled if possible. If you are unable to do so, then there is no blame upon you due to your ignorance of the Sharia ruling, and ignorance absolves one of sin.