This hoverboard doesn't have any unlockable upgrades.
Sweetheart is a limited hoverboard in Subway Surfers. It was released with Subway Surfers World Tour: Paris 2018. Like any other hoverboard, it will protect you from crashing for 30 seconds. When crashed, the board will explode and recharge for a few seconds before it can be used again. The hoverboard has the appearance of a pink cloud.