Sure, when you set up your rule for WCS in your router it more then likely looked similar to this:
Name Start Port End Port Protocol IP Address
WCS 6689 6689 TCP 192.168.0.101
Now your IP address has change to say 192.168.0.102 then you will have to change this for the rule in the router, also note that you will need to change this for WinMX if you have ports set for that also.
This address is how your router knows which computer to allow this rule for, so if it gives your computer a new number the rule must match.