itch.io

According to this forum post, the only way is to do a refund and re-purchase at the higher amount. Seems like itch is planning on changing this in future, but who knows when that will happen. It looks like you'll have to contact the itch support to do this. Sorry for the inconvenience :(

I'm not really sure, I will look into that.

Yes, the save file can be transferred from the demo to the full game. And thank you for your kind words. :)

That's perfectly fine :)

What are your computer's specs? 

You'll need to be connected to the internet on the computer you're installing on as it downloads additional files. The actual install is about 4.5 GB total. Unfortunately I didn't have a very nice way of uploading that directly here without requiring users to download 7 different files (max upload size is 750 MB). If you can download it on another computer I can make an installer that won't require an internet connection for you. 

Go for it!

No worries. That's what makes this bug so hard to track down. It did give me some ideas to check into though, so hopefully I can sort it out.