Vodafone sees this as a way for iPhone owners to enjoy having access to Apple's latest and greatest smartphones each and every year, but the terms of the Annual Upgrade Promise don't explicitly limit customers from upgrading to a new iPhone.
Provided the cost of the new handset and plan you're switching to cost the same or more per month than you're existing plan, you can upgrade to any eligible new phone without paying an upgrade fee.
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Of course, if the phone you're upgrading to is worth more than the equivalent phone you're currently using, you pay an upfront cost for that new device as you would if you were picking the plan up anew.
It's also worth noting that this is a trade-in offer, so you will have to hand back your current phone and most importantly, it has to be in good working order (Vodafone details what it defines as acceptable on the AUP's terms and conditions page).
On closer inspection, what Vodafone's Annual Upgrade Promise actually is, is a means of letting customers on 24-month plans, start a new plan every 12-months - new phone and all. This does also mean that if you don't want to upgrade your device or plan, or you want to end your contract with Vodafone, you'll still have to see out the remainder of the 24-month plan you agreed to or pay a fee to end it early.