MIDI doesn't work the way you think it does. MIDI devices don't send out a message saying, 'I'm now playing on channel 1'. Rather, *every* message sent (with a few exceptions like SysEx that you probably don't need to know about now) is sent on a specific channel. It had to be done that way so that a single MIDI device can receive a merged signal from multiple keyboards and use the MIDI channel to differentiate which notes trigger which sounds, and also so that a single device could send MIDI data on multiple channels at once.
Bome's MIDI Translator will let you map keys to specific MIDI events. It might let you rechannelize all incoming MIDI data. Also, depending on the software that's receiving the MIDI data, it might be possible to use your computer keyboard to send MIDI CCs, again through Bome's, that mute and unmute certain tracks in your sequencer rather than rechannelizing all incoming MIDI data.
Unless you're playing live, I wouldn't bother with this. For sequencing and recording MIDI data, it shouldn't be a big deal to change.