Dynamic swapping of DDDWs

I need a fresh pair of eyes to see what I'm doing wrong:

Powerbuilder 11.5.1 running MS SQL Server 2008

The relevant window has a datawindow dw_1. The column reason in the satawindow is a dddw. I want to change the column to point to another dddw according to the usergroup of the user - certain values will be filtered out by the new dddw.

My code:

// original dddw  d_adm_commlog_reason

IF g_str_user.s_role_name = 'STAFF' then

   s1 = dw_1.Modify("reason.DDDW.Name = 'd_adm_commlog_reason_csr'")    

l1 = dw_1.GetChild("reason", idwc_reason)

END IF

s1 returns an empty string, L1 returns -1. The dddw is not populated after retrieve.

I have also tried

dw_1.Modify("reason.DDDW.DisplayColumn ='cod_desc'")

dw_1.Modify("reason.DDDW.DataColumn ='cod_code'")

ven though the display and data columns are the same for the two DDDWs. These commands return empty strings.

Thanks,

Nigel