About a year ago I purchased a game with bonus content at a certain price. It recently got updated and it's no longer in my downloads.

Is it possible to pay just the price discrepancy to get the extra again or do I have to buy the entire game again? Feels like by now there should be a way to do this.

Feels like by now there should be a way to do this

There is. Steam does it like that. But I think some of those bundles might be made by Steam and not the publishers. They also clearly tell you how much you are saving. That is a bit unclear with itch bundles, because itch bundles are just that. Bundles. There is no discount, unless configured. And when the individual items are on sale, you might miss that the bundle actually has no discount.

Itch could also start with giving ability to increase payment level, instead of making people buy twice, when they try that. Or overhaul the individual priced files mechanis as a whole.