Do I need to be buying a new device to get trade in value?

Apple does have a trade-in scheme. You can check eligibility - and obtain an estimated trade-in value - here:

Apple Trade In - Apple

If using the online trade-in, you will be sent a shipping kit for each device. Devices are returned, assessed and a credit value assigned. The trade-in value is returned to you by way of an Apple gift card for each device - and these can be used to purchase whatever you wish with Apple. 

If a trade-in is initiated during an online purchase, you will usually pay full price for the item - but will receive a refund against your credit card upon subsequent return (and acceptance) of the item that was returned for trade-in.

Many choose to complete their trade-in in-store - as doing so removes any uncertainty as to the trade-in value or acceptance.

You can trade in, get a gift card, and spend it later.

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