"If it ain't broke"

[Exiled Yorkshireman]

I agree with the "if it ain't broke, don't fix it" approach to electrolytics. A bit of physics and chemistry helps to  understand what's going on. The dielectric in electrolytics is a thin layer of aluminium oxide (the dielectric) on one of the aluminium foils which is one electrode. The electrolyte (in conjunction with the other foil) is the other electrode. If the electrolyte goes AWOL, then the ESR will go high, and that's a sign that the cap is knackered and should be replaced. The more electrolyte that goes missing the lower the capacitance, too.  Now most people think of aluminium oxide as being insoluble, but actually over time, without any polarising voltage applied to the cap, the thin layer of oxide does dissolve back into the electrolyte. Of course, the thinner the dielectric, the higher the capacitance, which is why we often measure the caps as being high. Also, the thinner the dielectric, the lower the breakdown voltage. If the oxide layer disappears entirely, the cap will go short and is beyond retrieval. Caps that have not been used for some time need to be reformed slowly, without exceeding the present breakdown voltage. The gradual application of the polarising voltage will slowly re-form the oxide layer by electrolytic deposition, the leakage current will decrease, and the capacitance will drop. The key is to keep the current down to avoid local heating and to keep an eye on it. I generally find that if the leakage current is stable and within reasonable limits at the working voltage then the cap is good to go, other measurements being OK, of course. 

Another reason I do this is 1) I'm a cheapskate and 2) I'm cackhanded at restuffing.