The people who bought before the bundle was updated would need to return to the page to download the updated version.
You sure? OP said the opposite. It does not work this way. The projects in a bundle are frozen at the time of purchase. If you add projects to the bundle later, those do not exist in the updated version of the purchased bundle.
It probably is due to this:
Every purchase or donation comes with a unique download page assigned to your transaction.
You will get a unique page containing purchased items from that bundle - from the moment of your purchase. You will not get the bundle page itself as a purchased item. From your purchase history you will see that you have bought the bundle. But the page you will see, is not the current bundle page, but your own unique download page.
So this is bad (or good), if you re-use bundle pages to release something like a vn in chapters and maintain a bundle that holds all chapters at a discount (you would add new chapters and increase the price of the bundle). People that bought chapter 1,2,3 in the bundle would not be able to later access the new chapters. But they would also not be able to purchase only the new chapters with a discount. If one intends to release something this way, it would be better to use the workaround OP detailed. Doing it on a single project page, and not with bundles. But it depends on what you want to actually achieve.
Itch does not handle duplicate purchases well. In contrast, if you buy a bundle on Steam, it will refund any duplicate items. You can utilize this, if a bundle has a discount and you already own most of the items in the bundle. The remaining items will have the bundle discount and you will only pay for the remaining items.
While Itch has pay what you want and those individually priced files, they do not have a way to increase your purchase tier, like buying it twice. And I am still unsure, how the purchase tier is handled, if the game was bought in a bundle or at a discount. Or a discounted bundle with discounted items.