Yep, just name the upgraded weapons the same. Instead of Wood Sword becoming Steel Sword, you can name it Wood Sword as well. You probably want to name them different things for a while though, just for debugging. It would be difficult to try to upgrade Steel Sword to Steel Sword to Steel Sword 10 times and keep track of the thing. At least number them for your own sake, then go back and take out the numbers (Steel Sword 1, Steel Sword 2, etc.)