Subscription upgrade

Hi. could somebody help with an issue i have with upgrading subscriptions. i had a 1 device subscription that i paid yearly. i am only 6 months in to that and want to upgrade to cover 2 devices yet i am asked to pay £80, the full amount for multiple devices. what happens to the remaining amount i have already paid for the lower prices subscription. is this refunded? i expected to pay a reduced rate of the higher subscription.