NuYawka,
Have you tried editing the XML for the channel numbers? I have reordered the channel numbers by modifying the XML however, I haven't tried replacing a channel number. However, if you want to try it,
Click on My Activities.
Click on Modify the XML...
Click on the Cable Box Activity
The XML will be displayed.
Find HBO channel 400 in the XML and change to 100. Save changes and download to your remote and test. This is only a temporary fix because the next time your remote is updated the channel number will probably be overwritten. However, maybe the guide will be updated by then. You may also be able to add the channel manually when selecting what channels are hidden and the favorites. I know there is a tab to allow adding a channel.
Hope this helps!