If the voltage to the capacitor is still safely below 400v, then there's no problem. Usually, you can allow for higher voltage capacitors as this does not usually cause any problems. If you use a capacitor with a lower voltage rating, it's only okay if the applied voltage is still a safe way below the rated voltage, otherwise the capacitor will likely blow up.
Only thing to keep in mind is, as the voltage rating of a capacitor increases, the ESR tends to increase, although this isn't a strict rule and is more importantly dependent on the capacitor.
Hope this helps.
Tahmid.