Hello.
I still don't have a definitive answer to that question.
You will most likely have to rebuy it, but I'm not sure what is the price.
The Itch dashboard doesn't give any answer to that question. However, some components are worded in a way that makes me believe you should not have to pay full price. If you bought the game for 5 dollars and it is now 15, you should be able to "upgrade your access" with just 10 dollars. However, I didn't try it and cannot promise it will work.
If you try it, please, let me know if that is effectively how it works.