make it possible to upgrade from a basic license to a pro license
In some cases, I might want to buy an item with the basic license to try it out, and then upgrade to the pro license if I want to use it. It would be nice to only have to pay the difference in price.
For example, if the basic license is $5 and the pro license is $20, make it possible to pay $15 to upgrade from a pre-existing basic license to a pro one.
If this is already possible, you should probably mention it on the license page, because I couldn't find any information about it. If not, I would really appreciate it if you could add this feature at some point in the future.

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Renato commented
I have to second: "Please seriously consider this request."
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anonymous commented
I just sold my first asset (Basic license) and I thought, what if they want to use it in a commercial game? Would they have to pay the full Pro license cost? I really am not comfortable with that for a few reasons.
1. There is no real way to protect our basic licensed assets from being used commercially. So making it easy for a basic license purchaser to upgrade later is a great benefit. It helps to keep them honest while still allowing them to demo the product at a reduced cost.
2. I believe that 'conversions' from basic license to pro will be much higher with a softer price point (only paying the difference).
I really don't foresee many, (if any at all) basic license purchases converting to pro licenses if they have to pay the full cost. Considering that a Pro License cost at least as much as a Basic license, they are paying over double the cost of a Basic for the Pro if they want to demo it (like any informed customer should do). Please seriously consider this request.