How do I get a refund on unused portion of AppleCare+, if I upgrade to new iPhone?

See the following for information about cancelling and refunds: How to manage your AppleCare plan - Apple Support

Not true Jonathan. Apple will refund the unused portion of an AppleCare plan.

Hi there,

I believe that the option is non-refundable or transferable, and that the service can not exactly be “cancelled” as such.

Hope this helps.

You don't.

The warranty stays with the device.