Need help with REST authentication - Unable to switch user authentication

This is my first project with SOAPUI (recently upgraded to Ready! API 1.3.1) and I think I must be missing something simple but cannot figure it out for the life of me.

I created an Authorization Profile for my REST project containing valid credentials for my user.  I have been assigning this profile (via the 'Auth' tab) to all of my teststeps and everything has worked fine (regardless if I use pre-emptive auth or not). 

The problem is I now want to create a teststep that uses creds for a different user (specifically a user that either doesn't exist or doesn't have access to my REST resource).  I want to verify our API returns proper error for these cases.  However, no matter what I try, the teststep keeps returning data from my valid user.  Seems as if it is caching the valid creds from my Auth Profile.  I've tried the following methods without any luck:

1. Create new Authentication Profiles with both valid user with bad pw and totally bogus user/pw.  Then assigned this profile to the Teststep via the Auth tab.
2. Set the Auth tab to 'No Authorization' and created a groovy script that assigned the user/pw to this teststep.  Script appears to run correctly.  I included it below.
3. Set the Auth Tab to 'Basic' authentication and explicitly entered bogus username and pw.

When I look in the HTTP log, I see the Authorization token and regardless of which user I am trying to use, the token is always the same as that of my valid user.

Here's my groovy script (copied and tweaked from this forum):

def password = "ILuvCarrots"
def username = "Bugs.Bunny@mydomain.com"

def testStep = testRunner.testCase.testSteps["Invalid User"]

testStep.setPropertyValue("Password",password)
testStep.setPropertyValue("Username",username)
log.info testStep.name +" password has been set to "+testStep.getPropertyValue("Password")
log.info testStep.name +" username has been set to "+ testStep.getPropertyValue("Username")

Any help or tips greatly appreciated.

Regards,

Greg