Trading-in my iMac

You will need to ask Apple that. We are all end users like you and have no idea.

However you should ALWAYS have a Time Machine backup of your computer, either that or a clone stored on an external HD. If you do not PLEASE backup ASAP or you will lose data!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

When the new machine arrives, before turning it on, simply connect the old Mac's Time Machine backup (or bootable clone backup) to the new Mac using a USB cable. Setup Assistant will ask if you are migrating from another Mac, select YES and follow the on-screen instructions. This is the quickest and most reliable method of migrating. NEVER EVER use Wi-Fi and connecting using an Ethernet or Thunderbolt connection is expensive and not as reliable.

You will need to check with Apple and/or their current third party partner - we are all other users here. FWIW, as far as I know, if you are not sending in the old one until after you get the new one, you will only get a gift certificate for your next purchase, so it probably would not matter all that much.

I am assuming that you have arranged to trade in your old computer when you bought the new one. If so, you will receive complete instructions as to how to transfer your files, clean your old computer and return it to Apple. They will then be sending you a container for the return. If your question is not answered, I'd check with Apple directly.