Line voltage capacitors generally consist of a thin plastic film coated with a very thin layer of aluminium, the whole being coiled up.
In time the plastic film breaks down and causes a local short circuit, this does not render the capacitor useless as the aluminium is vapourised from the shorted area. This feature is known as "self healing" In time though the capacity is reduced as more areas suffer thus.
In critical applications, the lifetime of a capacitor can be greatly extended by use of one intended for a higher voltage than the actual supply. For example on a 277 volt circuit, use a capacitor intended for a 480 volt circuit.
If a capacitor is part of a lamp operating or motor operating circuit, then the correct value must be used.
If however the capacitor is simply connected accross the supply to improve the power factor, then no harm will come of useing one 25% or 50% larger than the original. This gives more margin for age related loss of capacity.