That API call (and CLI command) seem to have been depreciated. It looks like a two-part process for convertible instances now. You have to first call GetReservedInstancesExchangeQuote followed by AcceptReservedInstancesExchangeQuote.
Thanks for the observation. I just took out the reference to the ExchangeReservedInstance API call for now. I’m likely going to redo the lecture soon anyway to cover the new Savings Plan feature that AWS just announced as well.