How can I change the promocode through a script?

I have the following code running in a Client script:

var jpromo = nlapiGetFieldValue('promocode');
if (jpromo!='14')
{
nlapiSetFieldValue('promocode', '56', null, true);
}

What this is SUPPOSED to do is this:(all depending on the rest of the script that isn't there, but is working) Whenever a certain category of user is logged in, the promocode can only be two different promocodes. Values of those are indeed 14 and 56... 14 is selected in certain cases, other than that it needs to be 56. (that's the value, I know.. not the text).

Say the text for 14 is TEST1 and 56 is TEST2.
Now, if someone was able to get TEST3 in there, it needs to change this to TEST2. So far, my script disables TEST3 (not sure if that's actually set up somewhere else.. but ok), but the text of TEST3 is still in the promocode field in the web shopping cart. Is my script failing or did I miss something else...?