I would suggest to allow users to be able to upgrade the product via paying the difference at the specific product.

I.e. : Avatar Base Version - 10 bucks

Avatar Advanced Features - 25 bucks

Allow user to upgrade tier by adding additional 15 bucks as an automatic discount.

In addition, allowing Creator to mass-send deals for upgrades, i.e. "Hey You bough this product, we have this version of it, and just for You, the upgrade price is reduced by [number]%off if you choose to buy it in the [time_window]"