How to access <bindItems> using Formcalc script

I have a drop down list with the following:

<bindItems labelRef="MGTXT" ref="$record.DROP_LISTS.PROMOTION_RSN.DATA[*]" valueRef="MASSG"/>

in the XML Source listing of my form.

I want to change the binding using the Formcalc scriptiing language.

How do I write the script to make the change?