Hi, I hope someone can help me with this. This might be a simple question, but I can't seem to find the answer anywhere.

I have a buyer who have paid $25 for a project.  A few days later, I added an additional file that's priced at higher point $33.90. 

• I know that if new buyers pay 33.90, they get the original file and also the additional file. 
• Is it possible for people who have paid $25 for the original file a few days ago to pay an additional 8.90 to get the additional file?
• Can they just pay the project more to do that? What are their options? 

Your option and the buyer's option is to complain to itch about this feature. And you should. If enough people complain about this, maybe they finally fix this. It does not solve anything and only creates problems.

It seems to have been meant for one and only one situation: someone wants to pay for some content and ALREADY knows he wants to pay that bit extra to get the bonus content.

Individually priced files are botched completely. For one, they are not as it says on the tin, indvidually priced. And most importantly you cannot increase your payment. You will buy the same project a second time, instead of increasing your donation level.

Most devs use this feature to have a free game and a paid game on one project page - instead of having a paid game with a free portion on one project page. For the most part this works the same. But it majorly screws up bundles, discounts, price changes and whatnot. And it gets ridiculous if there is more than one tier. 

Or in your case, a paid game with an extra tier.

Add the individual file as an extra project as a dlc and not as a bonus file with higher tier. There will only be strife and anguish. No one understands this pricing scheme, not players, not developers and apparantly not itch, as they should fix this already, otherwise.

they get the original file and also the additional file

No. They will not. This is the crux of this and why this feature needs a serious overhaul.

They will have access to this file. They will not get it.

If you set a price for your project and someone buys it, they will get the project in their library as a owned project. They will be able to download it, even if you change the price and there are hurdles to take it away or remove the project later.

If you change the price level for that individual file to a higher level, the buyer will lose access to it.

The thing with the price change is important for projects that start out as free with pay what you want and set a minimum price later. Anyone that paid anything will still have access. Even if they paid 1$ and you set minimum to 25 $. It is as if they bought it in a sale for 1$.

And for those individually priced files, you cannotshouldnot change prices - ever. If you set it to 3$ as a bonus to a free game and later decide to change the tier to 5 $ you screw over all your previous buyers.