Substituting capacitors
The voltage is the maximum the capacitor can withstand before the dielectric breaks down, and does not factor into the capacitance (measured in farads, \$F\$). Use a capacitor with the same capacitance, and the same or higher voltage rating.

Also, \$Q\$ is charge, and although it is the product of voltage and capacitance, you must use the applied voltage, not the rated.