When you run a session, there's always at least one agent associated with it.
If you modify the agent while you are running the session, the results will be reset to Zero. You will have to re-run this session.
Results are associated with the session and also to the agent. So changing the agent is, in fact, like if you are tampering the results.
XStudio warns you using a pop offering you to dismiss your change before it impacts the running session.