I replaced the battery in a WarSong (http://en.wikipedia.org/wiki/Warsong)
cartridge once. It was a typical CR2032 and very easy to replace. Sometimes the batteries are soldered in, so you either have to heat the existing solder until it loosens (recommended) or cut the battery off along with a small piece of the conductor (not recommended). You don't necessarily have to resolder the new battery in, but if you drop the cartridge afterwards, the connection might break for a split second, and you will lose the saves, so get a soldering iron and practice.