If you select the old network in the wifi settings on a device there is usually an option to forget. Once you have selected this the old network should no longer be shown on that device. You will need to do this on every device that still shows the old network.
No double charging. VM only charge you for your BB connection, your wifi connections are just part of your home network. The old wifi channels will no longer exist now that you are using the new SH3. You just need to remove the reference to them in devices that used to use them.