This is the closest thing I could find.

1. Press the [UTILITY] button.

2. Press the [F5] button for “CONTROL”.

3. Press the [SF2] button for “MIDI”.

4. Cursor to “Keyboard Transmit Ch” and use the [DEC/NO] and [INC/YES] buttons to change the channel.

No idea if it is accurate since I don't have a Motif. The way #4 is worded makes me think it may not be right.