https://www.electronics-tutorials.ws/rc/rc_1.html https://www.electronics-tutorials.ws/rc/rc_3.html Changing either capacitance or voltage rating will alter multiple important aspects of the circuit. Time constant, inrush current, frequency cutoff, etc. Ideally, you want to use exactly the specs that were originally there. But again - staying within reason, I'm betting you won't notice any real difference (FOR THIS SPECIFIC APPLICATION). I'd bet a dollar that IF there is any noticeable difference, then 6.3V 1200uF will be uglier than 2.5V 2400uF because it's going to screw with the current sensing more. I'll toss a few different ratings on my next Mouser order to show you on the oscilloscope.