All perfect praise be to Allah, The Lord of The Worlds, and may His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
Zakat-ul-Fitr is an act of worship that depends upon the intention because the Prophet(pbuh) said: "The reward of deeds depends upon the intentions and every person will get the reward according to what he has intended." {Related by Bukhari}.
Intention should be made before paying Zakat to its eligible recipients or to the charity that delivers it to them. Therefore, making intention after payment isn`t permissible nor is changing the intention afterwards.
Al-Imam An-Nawawi (may Allah have mercy on him) said: "It is permissible to make intention before payment, in analogy to fasting, since the intention behind giving the Zakat is providing for the poor, and this is also the view of Abu Hanifa, and the apparent meaning of al- Shafi`i`s text on expiation, where he said: "Zakat doesn`t avail him save with making the intention upon payment or before it." Our fellow scholars said: "in terms of intention, Zakat and expiation are the same." {Al-Majmou`}. However, if you (asker) have paid that money to a charity then it is authorized to deliver it to the eligible recipients.
Therefore, if you want to change your intention then you may call the charity and ask whether it has delivered the money to recipients or not. If it has then changing the intention isn`t permissible, but if it hasn`t then it is permissible for you to change it, and it is always better that you inform the charity about your intention and goal behind giving the Zakat. And Allah knows best.