Once announced and if Sony repeats what they offered in 2018, the deal works through a participating local camera store. In the Seattle area, that’s Glazers and Kenmore camera. For the local store, bring your used working interchangeable-lens camera or lens into your local photo retailer for an evaluation. They’ll provide a quote for how much they’re willing to give you for your trade. If you agree to the amount, the next step is to choose the Sony full-frame camera you want to buy. If you’ve selected one of the bonus-eligible cameras, the sales associate will apply the corresponding trade-in bonus to your purchase, along with all other applicable instant rebates.
This seems a lot easier to do in person and Amazon didn’t participate, but B&H did.
If it’s up to $500.00 off plus the value of your trade in is that’s as good as deal as it gets; especially for the a9 after the firmware upgrade.