Can I trade in an iPhone for a tablet?

Yes, you can trade-in a qualifying iPhone (or any other Apple device) towards purchase of any other device - or can opt to receive a gift-card to be used against a future purchase from Apple.

You can check eligibility - and obtain an estimated trade-in value - here:

Apple Trade In - Apple

The available trade-in value that you see listed in the online trade-in calculator is the maximum estimated value - amd assumes that the iPad is undamaged and fully working. The actual value is only finalised when the qualifying device has been inspected and accepted. 

Be aware that online and in-store trade-in values can differ - however, many find that completing a trade-in using the in-store process removes any doubt as to the value offered; the transaction is finalised without the risk of shipments going astray, or disagreement as to the condition of the device when it was sent to Apple’s trade-in partner.

Apple's trade-in programme will only accept qualifying devices if they are fully functional and undamaged.

If you're doing it through Apple, only by getting a gift card and spending that on the desired unit. Otherwise, ask the company involved.

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You’ll get less than 10¢ on the dollar. Best to gift the iPhone to someone… makes them a you happy. That is what I do.