Usually 1-1024 are all reserved for important processes, and besides that you've got 64512 ports to work with. What happens when you try to bind to a port already taken? Your computer tells you you can't in some way (probably warcraft sends an error).
Changing the port number shouldn't make your computer less secure in any way. Some ports have a reputation for being backdoors but that's because backdoor services use that port, not because that port is cursed to do evil things.
Of course, I barely know what I'm talking about. I could have sworn that you could only bind to 6112-6119, but it seems I'm wrong. Not liable for whatever goodies and badies may happen to your computer because I said you're not gonna be less secure.