Power Suply

On my brothers computer he only has a 300 watt power suply, he were to get one and i was to help him install it, would i have to rewire everything? or just pull all the plugs off, take our the old one, put the new 1 in, and plug it in? All of the power cords are all stuck together with those clicky plastic things and all of them together are about an inch in diameter when they go into the power suply.