The bigger issue IMO is apple doesn't offer the ability to upgrade.
That's partly due to how the App Store works. One of the rules is a vendor can't offer upgrade pricing on an app to a newer or different version. The only way to get an improved version is to offer it as an entirely new app the user has to purchase. The only thing a vendor can do with an app you already have is to add whatever they want to it, but at no extra cost to you.
That's kind of splitting hairs with 365 since the apps are identical between the individual and regular plans. The only difference being able to install it on one computer vs. up to six. But the cost is different, so the App Store rules make it a completely separate purchase.