If you change your subscription plan to Premium Duo, the remainder of your individual Premium time from the gift card will be converted into Premium Duo and shortened due to the price difference between individual Premium and Premium Duo.
Bear in mind that once you change your plan to Premium Duo, gift cards can't be used to pay for this subscription plan, so you'll need to use a recurring payment method for future billing.
You can also read more about how Premium Duo works here, as well as in this FAQ article.
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