Yes, you can!
On iOS
Your Platinum subscription starts immediately upon upgrading, and your billing cycle will reset to the date of upgrade. You'll be charged the full Platinum price upfront, and the unused portion of your Premium subscription will be refunded on a prorated basis by the App Store within a few business days.
On Android
Your Platinum subscription also starts immediately, but your billing cycle stays tied to your original Premium subscription date — it won't reset. You'll only be charged the prorated difference for the remainder of the cycle.