Logon name Custom VB Script - Creation Fire on Edit/Change?!

We use a custom vb script to create a samAccountName (Logon name) for the user upon creation integrated in a policy. The template for the script I found somewhere on the one identity forum. it works well. Now I have found out, that if I want to edit / change an existing user logon name, the button with the little lightning icon does not fire my script. How can I make the button click fire my logon name generation script? I played around with "onpreModify" event handlers but had no luck. please check screens 1 - 4, so you know what I mean.

Script Screen1:


Script Screen2:


New User Logic works:


Exisitng User Logic not triggering: