the config file you need to edit is in
C:\Users\[user name]\AppData\Local\Britton_IT_Ltd\CrewChiefV4[some random rubbish]\4.9.3.2\user.config
(or whatever the latest version version subfolder is)
Do the assignment from the CC interface, and save it (even if you think it got the wrong button ID) - the device ID should still be correct.
Look for the button index you want in that config file and edit the value element by hand - e.g. CHANNEL_OPEN_FUNCTION_button_index
Code:
<setting name="CHANNEL_OPEN_FUNCTION_button_index" serializeAs="String">
<value>19</value>
</setting>
<setting name="CHANNEL_OPEN_FUNCTION_device_guid" serializeAs="String">
<value>6f1d2b61-d5a0-11cf-bfc7-444553540000</value>
</setting>
<setting name="CHANNEL_OPEN_FUNCTION_device_type" serializeAs="String">
<value />
</setting>