I managed to snap off one of the battery clip wires at the board of my Colorsound Wah.Can I just reheat the joint with a iron and reinstall the wire, or do I need to de-solder the old joint and apply a new solder joint? I've heard simply reheating a joint can lead to a cold solder joint.

I'd desolder it b/c it probably has some of the old wire still in there. But just reheating it won't necessarily make a cold solder joint. Make sure you have a good soldering iron with a clean tip, heat the solder all the way through but don't overheat it.