You can change the configuration of the column editor. Just like this:
var oid = $('#dgrid').datagrid('getColumnOption','ORDER_ID').editor;
oid.options.url = '/?someurlandparams';
Or call 'reload' method when beginning to editor a row.
$('#dgrid').datagrid({
onBeginEdit: function(index,row){
var ed = $(this).datagrid('getEditor', {index:index,field:'ORDER_ID'});
$(ed.target).combobox('reload','/?someurlandparams');
}
});