I'm very sorry to say, but setting the price of the new version above the old version requires a new purchase of the full value. I wish that the purchasing methods here at itch.io were more friendly but it's not merely "pay another $2 for the new version". It ends up charging people twice.
I understand not being familiar with the process. But there's no sign anywhere that a discount like that can be applied. You'll have to manually set the price of the new version back down to $7 so people aren't paying full price and some extra for an update.