Yes, the 100 volt electrolytic will eventually become a 50 volt electrolytic (or whatever voltage is actually impressed on it). The aluminum oxide film that is the actual dielectric of the capacitor will hydrolyze eventually dissolving the oxide back into the electrolyte. This is why relatively new electrolytic capacitors sometimes need to be reformed. In most cases, this is seen as an elevated leakage current in the first few hours of operation.
In the old days when vacuum tube rectifiers were in vogue, power supply electrolytics would be subjected to a higher voltage than their normal operating voltage until all the tubes were "warmed up". This is why high voltage electrolytics often had a surge voltage rating high enough to prevent breakdown during those few seconds that the supply was not providing current.
I wrote a number of posts about 9 to 10 years ago on electrolytic capacitors and their chemistry. Most of these can be found archived at
Dr. Barry L. Ornitz WA4VZQ