Exploring and Integrating a FRITZ!Box Fon into existing network

Problem: The IP of the LAN Interface is fixed to and of the USB Interface to
If you have an existing network, this forces you to renumber your network. This isn't the nice solution anyway, so I was searching for another one.

At first, I updated my firmware to Version 06.03.14. On many sites you can find that you can change the IP on http://fritz.box/cgi-bin/..%2Fcgi-bin%2Fwebcm?getpage=..%2Fhtml%....