Q: discount on future purchase

This is left at the discretion of the Apple Store. You can always call the store you are planning to go to and ask them if they offer in-store credit in exchange for the model you have. If the model is vintage or obsolete, they will likely not want it (see the link at the end of this message for a full list of models). Else, they will probably want to look at your device before they give you a price for it.

Vintage and obsolete products - Apple Support