The process is quick and easy, and you get the maximum value of the trade-in right away. If you decide to trade in your device to Apple, please do it in person at the Apple Store. So I think this info is useful for people who want to try a new device before they decide whether if they should sell/trade in their old device. I just finished traded in my old iPhone after 14 days since the day I have bought my new iPad mini, and they accepted that without any problem. It takes 3 days for the credit to be applied to your card. Moreover, they will ask you to give them your credit card/debit card so they can send the credit to that card.
When you bring your old device to the Apple Store to trade in later, just show the Apple Specialist the invoice along with the serial number of the new device that you bought, so they could apply the credit to the device in that order. You can do that within 14 days when you are satisfied with your new device. When you purchase a new device at Apple Store or on the Apple website, you don't have to trade in your old device to get the credit right away. Ask Appleās specialist to confirm this trade-in policy just in case they change it later. Trade-in your old devices at the Apple Store within 14 days of a new purchase. If you choose the mail-in option for online trade-in, the Apple store will refuse to process your trade-in at stores. Therefore, you should ask Apple's Specialists first to confirm whether if they allow you to trade in your old device later. So when I went there yesterday (13 days later), they processed it right away without any problem. When I picked my iPad mini 6 at the Apple store (in the US), I asked Apple's Specialist whether if I could trade in later, and they said yes.
Some people said that Apple refused to apply their trade-in value to the original purchase. Maybe this policy is different among stores.