I have a circuit that is calling for a 400v capacitor to be used. I only have 1000v available. Can I susbtitute it and use the 1000v?
The volt rating on a capacitor tells you at what voltage a capacitor breaks down. When it breaks down the capacitor will burn through its dielectric causing a short. There is nothing wrong with using a capacitor with a higher volt tolerance, just make sure the capacitor has the same capacitance and that both are polar or non-polar capacitors.
Yes (assuming that all other parameters of the capacitor are the same or otherwise acceptable). The voltage rating of capacitors, called the breakdown voltage, is the maximum voltage the capacitor can safely handle.