It makes no consistent difference Call vs Chat, the retention offer is generated by an algorithm, not by the agent. See here:
American Express retention offers (2022)
There is no bonus for downgrading. You would need to apply for a new Green, Gold or (another version of) Platinum.
Thanks mia. I chatted with them and they didn't have anything to offer me. Bummer but such is life. I will post in the linked thread as well.