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.