The environment I am scripting in uses server-based authentication (claims based).
How would I get the Users to change between iterations?
I have tried a Variable Modifier but I don't have visibility on the change.
There is no way, as far as I can find, to change claims based authentication. In my case I was also working on a Kerberos authentication solution with a one hour time out on the tokens issued. My solution was to have staggered populations starting up over the period of a test and to sustain staggering for the duration of the endurance test.
At the Actions container level there's an option to simulate a new browser between each iteration. That will remove the user context but not sure if it will also remove the authentication token. It's worth a try.