Is it permissible to use money designated for the poor and then replace it with other money, and is it permissible to change the intention of this money from charity for the poor to building a mosque?

A gift is not binding unless it is taken possession of. Therefore, there is no harm in retracting it as long as the donee has not taken possession of it. It is not obligatory to give a substitute, but it is recommended. There is no harm for someone who intended money for the poor then spent it on building a mosque, as long as the poor had not taken possession of it.