thats what Xayma said, i think, you misunderstood his post. im a expert at electron theory of electricity.
i would just like to point out that electricity is a flow of charged particles, ie protons, electrons... and not just electrons. but in common metals its 99.9% electrons because of its delocalised electrons.
the particles actually move very slowly, but some books claim that they move at the speed of light, but this not true. its the energy ie the potential difference induced force that moves at the speed of light.
so to answer the original question. just think the metal wire as a pipe filled with water already. when you pump water from one side to the other you have to create a potential difference in amount water in the 2 sides ie the end of the pipes. and after some time then you stop this by blocking the sides, there will still be the same amount of water in them pipe, except now they are not the same water as they were at the start. so yes they get replaced. but this electrons don't actually move very fast, its the speed at which the signal of the water movement is being sent that travel at the speed of light.
hey lil tiger, do you do chemistry? if you do then, i think what you were after is an electrolytic cell. its used in many different applications to change states of substances ie metals for industrial purposes by the use of external power source. you should just read text book... can't be bothered to explain everything.