I'm not sure what you mean by replace the character? You can make a copy of the project file, open that, and edit it to make a new character. Or you can duplicate the skeleton, as Spine supports multiple skeletons within a single project. With both of those, you'll have 2 copies of all the bones, animations, etc.
If you use skins, you can use the same animations for both characters. You can use skin bones to have bones that are only visible for some skins. Unless the characters are very different, either with their rigging or their animations, skins are probably better.
