Yes, you can!
On iOS
Your subscription starts the day you upgrade, so you can enjoy all the benefits of Platinum right away. Your subscription cycle will also be reset. For example, if you upgrade on the 15th day of your Premium subscription, your subscription cycle will reset and be renewed based on the day you upgraded.
You'll only pay the difference in pricing, but you'll be charged the full price of a Premium subscription and receive a prorated refund a few business days later.
On Android
Your subscription starts the day you upgrade, so you can enjoy all the benefits of Platinum right away. Your subscription cycle will remain the same as your Premium subscription. For example, if you upgrade on the 15th day of your Premium subscription, your subscription cycle will not reset and will be renewed based on your Mini subscription date.
You'll be charged the prorated difference for the Platinum subscription.