If you found an e-cap bad and could not locate the same exact value, usually you will look up for a replacement that have higher voltage-am i right? For example, you replace a 470uf 25 volt with a 470uf 35 volt capacitor. The question now is could you replace the e-cap say with 470uf 150 volt? The answer is for electrolytics, the operating voltage should not exceed by about 50 % the working voltage of the original cap, or the circuit voltage. An e-cap with a much higher voltage rating may, over time, decrease its amount of capacitance when operated on a considerably lower voltage. Technically, the capacitor cannot properly form itself at the lower voltage , and loses capacity and this cap will give you problem down the road.