What do you mean, specifically, by "It won't let me change it"?
The "Override Joystick Recognition Button" option of the active profile should apply.
If you want to use multiple different joystick buttons to each independently enable listening (I.E. not as a combination), and you're using any option except "Listen then stop listening after recognition completes", you can create a command containing the appropriate combination of "Make VoiceAttack Start Listening", "Make VoiceAttack Stop Listening", and loop actions.