Is it possible to substitute a vow of i'tikaf (spiritual retreat in a mosque) with fasting or charity after the hardship for which the vow was made has been removed?

A vow must be fulfilled and cannot be changed. If you are unable to fulfill it due to a temporary impediment that is expected to be removed, then wait until it is removed. However, if the inability to fulfill it is not expected to be removed, then you must offer an expiation for an oath (kaffarat yameen). The expiation for an oath is: freeing a slave, or feeding ten poor people, or clothing them. If one cannot find [the means for] these, then he must fast for three days.