Q:

Update the AccountUser Object

Is it possible to modify or update the AccountUser Object using the ClientID and ClientSecret credentials or do I need to use an API User that is set-up in the account?

A:

You can use ClientID and ClientSecret.  You need to get the token first, then put as your soap header:
<soapenv:Header>
  <fueloauth xmlns="http://exacttarget.com">TOKENGOESHERE</fueloauth>
</soapenv:Header>

Good luck!

