ANSWER:

No this is not permissible. You cannot change the intention now, intention should have been prior to commitment. 

Some money was being collected as Sadaqah for a brother. I committed to an amount to the one who was collecting, and he paid the amount to the brother (Money still with me).Later it was told to us that the brother is eligible for Zakah. Can I ‘change’ my intention to have this counted as Zakah from me, coz earlier I didn’t have this intention as we didn’t know.