It will have less energy when discharged, so theoretically, by E=mc^2 it will have less mass and therefore weigh less. However, the difference is so small you would be unlikely to detect it even with extremely sophisticated equipment.
Yes of course, thanks for that. I was thinking in the line of loss of electrons in capacitor and hence weight loss. I guess it follows from E=MC^2 that a wound up spring would theoretically weigh more also.
You are welcome, however I think you may have a misconception about what it means to charge a capacitor. When a capacitor is charged there is no net charge, instead there is only charge separation. A small number of electrons are removed from one plate and the same number of electrons is added to the other plate. The total number of electrons does not change, only their location.