Change label

Hi,
I would like to be able to change the label for a edit control dynamically before the page is displayed.
I the following code I have tried using
$xt->assign('label participant p_course_site_id', 'new label'); - in the Before display event.
<TR class="{$fielddispclass_p_course_site_id}" data-fieldname="p_course_site_id">

<TD class="rnr-control style3" rowspan="1" colspan="2">

<SPAN class="prk-label">

{BEGIN p_course_site_id_label}

{$label participant p_course_site_id}

{END p_course_site_id_label}

</SPAN>

<BR>

{$p_course_site_id_editcontrol}

&nbsp;

</TD>

</TR>
It is not working. Any Ideas?
Kind regards
Thomas
Thanks!