First, you will need to update your TiVo to the latest firmware before it will detect the correct IR Codes for the VIP 1200.
Once you have the latest IR Database, version 243 or higher, it has codes 10081, 10082, and 10083 which allows the TiVo to change the VIP1200 channels correctly.
The problem you will find is that the serial IR connectors are very weak in changing the channels and will give you trouble. I recommend one of 2 things:
1) Tape plastic knives to the IR Sensors, place one on the top of the unit over the USB port, and one on the bottom below the USB port. Altneratly, try placing them where some people think the sensors are (near the link led)
2) Go buy an IR Blaster that is compatible with the TiVo. I purchased one from smarthome for $12.99 that has double sided tape on it, which worked pretty well for me.
Good luck!