|I assume, we're talking about a wireless router and you allowed others to connect to your router wirelessly.|
You don't have to change the ip address of the router, but the encryption key.
Read the routers manual on howto change this.
You also have to change the encryption key at your own wireless devices, to further get them connected to your router wirelessly.
The original poster should always write the last response !!!
Let us know, if the problem is solved !!!