Upgrading to AppleCare One with active AppleCare+

I paid for AppleCare+ upfront on my headphones and laptop (they each have about a year of AppleCare+ left). I just got a new iPhone and am considering upgrading to AppleCare One to cover all three devices. But when I click terms and conditions, it says "We cannot currently provide you with an estimated refund, but if you are due a refund, you agree to receive it via email." How can I find out how much the refund would be and how it will be credited?