My experience is that the way you have set it up, I'll need to pay the full price - I bought this for 3 years ago. PonyDragon seems to know what he is talking about, perhaps there is something you can do reading his answer ?
Unfortunately, the faq does not really say either way. https://itch.io/docs/creators/pricing#purchase-tiers-through-individually-priced...
The missing information is, that multiple payments are not added together for the unlock price. But even if that were so, that would only be "ok" for additional dlc and not for a price raise of the main game.
Developers using that mechanic to set the price for the item and raise prices later do something unexpected. Bought digital items stay bought. But this file price mechanic does not buy the files, it only compares the unlock price with the file price.
The game is not out of early access yet, is it? If price changes again for full release, the same issue occurs again, if the price is set for the file and not for the project.
I see five solutions.
Make game paid, remove file price. Pro: previous buyers are still owners. Con: supporters with $ 1 are also now owners, but this can be seen as buying on pre-order discount.
Have old versions that were last available at that unlock price be still available and have new version at higher level. Pro: people get what they paid for. Con: confusion about pricing, customers might have expected differently. Trust loss for future price increase.
Make second project that is paid with updates. Freeze this page at certain versions and make clear it is the support version with no updates. Pro: the paid version could include a Steam key. Con: making people buy again for new version. But right now this is also so and previous buyers at least can access what they paid for.
Continue pricing scheme. Focus on Steam. Let Itch customers buy again and then again when price rises a bit in the future. Pro: more money, since customers readily buy things again Con: some people might not like this.
Some manual solution and a combination of the above mentioned options. But whatever it is, the price of the game should not continue to be a file price. Pro: tailored solution to make everyone happy Con: might be a lot of work, depending on how many people would be sent an Itch access key and how this would be done. I have seen devs do similar things in comments or on discord.