Currently I need to change the name of a column depending on specific criteria but to do that I'd like to refer to that column by its ExternalName rather than its name.
aColumn = Document.ActiveDataTableReference.Columns["I_id"].Name
unfortunately this doesn't work.
aColumn = Document.ActiveDataTableReference.Columns["I_id"].ExternalName
you're very close! ExternalName isn't a property of the DataColumn object, which is, I suppose you've figured out, why your approach isn't working.
in fact, ExternalName is an item represented by the DataColumnProperties.DefaultProperties class. you would actually access this as if it were a custom-defined Column Property like so:
col_ext_name = Document.ActiveDataTableReference.Columns["I_id"].Properties["ExternalName"]
print(col_ext_name)
>> index_id