If I make a change to the parameters of a Retention Policy, how does that impact messages archived under that policy?

Changes to the name of a retention policy will have no effect on existing or future emails archived under that policy.

Updating the criteria under which a policy will retain messages (e.g., if that policy will Archive messages based on contents within the subject line, the sending address, body text, etc.) will not impact messages already archived. Once the criteria for a retention policy has been updated, the Archive service will begin using that new criteria to determine whether or not new messages match that policy and if so how long they should be retained.